Taric really can't handle Nami's poke or her extra range. Use Ebb and Flow to keep him too low to initiate and aqua prison on his carry if he uses dazzle on yours.
Soraka can't really handle constant poke well and her low mobility gives Nami a huge advantage in this match-up
Morgana can block Aqua Prison and Tidal Wave with her Black Shield, which can be somewhat problematic. Early in the game, Ebb and Flow can be used to poke through Black Shield, plus Morgana doesn't handle poke well, which gives Nami a slight advantage in lane. Watch for Dark Binding, that will almost always mean a death for you. Post-six, this match-up goes more in Morgana's favor, but good use of your CC can go a long way towards dealing with that.
Sona has a decent amount of poke, but her lower range, innately low mobility, and squishiness give Nami an advantage in this match-up. As Sona 2.0, you pretty much win this match-up outright. Post-six, watch out for flash > crescendo.
Alistar's low mobility makes this a fairly easy match-up in lane. However he can basically ignore Aqua Prison post-six with Unbreakable Will, which can make peeling him off allies in a teamfight rather difficult.
Easy enough for you to poke and kite around. He'll have trouble getting off any meaningful plays if you play the match up properly.
Karma can poke through Nami's sustain and is mobile enough to avoid Aqua Prison and Tidal Wave. Try to avoid Inner Flame and poke with Ebb and Flow as much as possible. Post-six, this lane tips more in Nami's favor.
Play passively, Nami really can't heal through his poke and he outranges her enough to harass without worrying about Aqua Prison. Avoid Plasma Fission and Tectonic Disruption and use Tidal Wave to interrupt Life Form Disintegration Ray if possible.
While Lulu has poke, Nami has enough range and sustain to have a slight advantage. Be wary of help, pix! + glitterlance combo. Her ultimate can make it hard for post-six initiations to result in kills, so watch out.
Janna has no counter poke or any hard CC to really deal with Nami, she's also squishy enough that poke can be a real issue for her. She's mobile enough to dodge Aqua Prison however, and gives her carry enough mobility to attempt to, which can make this match-up kind of a stalemate. Her teamfight capability is better than Nami's, so that can turn the tides a bit out of the laning phase.
Nami's poke and greater mobility give her an advantage in this match-up. Avoid Death Sentence (its name is fitting) and think outside The Box post-six. If you miss an Aqua Prison and he can get close this match-up can quickly fall into his favor, so be careful.
His poke and CC are annoying, but you have more range and sustain. Not too difficult pre-six, but Tempered Fate has a huge range and can be much harder to dodge than Tidal Wave.
Nami is squishy and less mobile than she used to be. If Leona can catch her she will just die, but Leona doesn't necessarily handle poke well. As long as you can stay out of her reach, you can pretty easily win this match-up.
Blitzcrank can't really handle Nami's poke or her range. However, a proper rocket grab will most likely result in a death, so watch out for that.
Zyra's low mobility and innate squishiness makes this match-up a little easier to play, despite her poke easily eating through Nami's sustain. She can have trouble dodging Aqua Prison, but avoid getting hit by Grasping Roots or you'll get bursted down.
Really frickin' obnoxious. You can't stop him from pushing the lane into you and his damage can be higher than you can easily sustain if you're not careful. Post-six, it gets even worse, though the saving grace is that if he ults your carry you have two ways to easily interrupt that.
this match-up is incredibly difficult for Nami, play passively and try to bait unbreakable before using tidal wave in a teamfight.
His poke mostly outranges yours and so does his CC, this match up just plain sucks. He also has a massive amount of burst post-six, which you have very little counter to besides trying to exhaust him.
Flash is generally a core spell, there's no reason to switch it out for anything. Exhaust can be a stronger pick throughout the game, as it gives you a massive damage debuff, but Ignite can be picked up against a high sustain team or against a high sustain support like Soraka (though in the latter situation it's largely unnecessary).
As Nami can be very squishy early, I prefer to run a combination of health, ability power, armor, and mana regen to make her somewhat more durable and sustainable. I tend to harass frequently, so magic penetration marks are what I prefer for damage runes.
Nami is one of my favorite supports in the game currently. Ironically, I used to really dislike her, though around halfway through season three I picked her up because I got bored with Sona. She's currently one of the stronger utility supports. She excels at a combination of poking and sustaining, while also having strong CC early and great kiting potential. This makes her extremely variable and a strong pick with most carries.
Nami is one of the strongest utility supports at the moment. She has some of the most cost efficient sustain in the game, her poke is very good, and her CC makes her a pretty safe pick in lane. She is also a force to be reckoned with in teamfights if you know how to engage with Tidal Wave.
Nami is extremely squishy in lane, this makes Nami somewhat vulnerable to poke or high burst supports. While Ebb and Flow is extremely cost efficient, it still has a high mana cost. This means she has to be careful just how often she spams it, or she will be very vulnerable. Aqua Prison usage can often make or break a lane for Nami, this can make it easy to abuse her, as missed Aqua Prisons are largely wasted mana on a champion who already has resource problems.
-Very squishy early -Can have resource problems in lane, especially if Ebb and Flow is maxed first -Ability to utilize Aqua Prison can make or break a lane with Nami.
Surging Tides: This makes Nami an exceptional kiting champion, as she can easily boost her allies' or her own movement speed.
Aqua Prison: This skill requires a certain amount of skill to land, but is extremely effective if used properly. As it can stun multiple targets, it can be very effective at pinning targets. It can also be used as an escape/peeling tool, as you can drop it on top of yourself or an ally to stun a target who has jumped onto them and give a movement speed buff at the same time. The main downside is that it can be predicted fairly easily and dodged, as the skill does not have a particularly fast particle.
Ebb and Flow: This skill is extremely good for counterpoking as it deals quite a decent bit of damage, even early. It's one of the most efficient healing skills in the game in terms of mana used for health returned. However, despite how efficient it is, the mana cost is still fairly steep and you have to be careful not to spam yourself out of mana or you'll find yourself vulnerable.
Tidecaller's Blessing: Combined with Surging Tides, this makes kiting extremely easy. The magic damage buff is also very nice for adding a little more damage to trades. The mana cost increase is not very efficient when compared to Aqua Prison though, which makes it less of a priority.
Tidal Wave: Combined with Aqua Prison, this skill is very effective for initiating fights. The slow also makes it good for escaping/chasing, as its slow duration increases depending on how far away the targets are from the skill's starting point. The skillshot is wide, but it can be dodged and must be placed properly to be effective in teamfights. Its low cooldown (for an ultimate) is fairly forgiving, however your allies may be less so if you mess this skill up very often.
Currently, maxing Ebb and Flow first is the best option on Nami as it gives her strong poke and sustain all in one handy skill. I usually prefer to max Aqua Prison second (Xpecial seems to also use a similar skill order sometimes), as the decreased cooldown and static mana cost allow me to make more plays more often. I then max Tidecaller's Blessing last, as the increased damage is often not worth the increased mana cost when compared to the reduced CD of Aqua Prison. Tidal Wave should be taken whenever it's possible to take it.
Either of these are perfectly good starting items on Nami, it all comes down to playstyle. I generally prefer Spellthief's Edge as it fits my playstyle better, but Ancient Coin is okay if you prefer to play more passively or are in a difficult match up.
As some type of Sightstone upgrade is core on supports in general, mentioning it seems almost redundant. However, the health Ruby Sightstone provides is quite beneficial on a squishier support like Nami, while Eye of the Watchers gives you a bit better slot efficiency and a decent chunk of AP. Redemption is a core buy on most supports and Nami really benefits from everything about the item. Oracle's alteration is another core support item, that should be bought fairly soon after hitting level 9, as it allows you to gain vision control easier, which is important for keeping an eye on objectives such as Baron Nashor and Dragon.
These are the best defensive purchases for Nami currently. Mikael's Crucible is very strong against compositions with a lot of hard CC, it can save an ally from an untimely death if the active is timed properly. Locket of the Iron Solari should be purchased against teams with a lot of Magic damage, especially if it's AoE, but is strong against any high burst team compositions due to the shield active. Randuin's Omen still remains the most efficient armor item for a support, as it provides health for its armor. This means the item needs very little health within the build to be effective, while Frozen Heart requires more health within the build or the armor is largely wasted. Zz'Rot Portal is a pretty decent purchase if your team is getting pushed in or if you want to start an AFK push in one of the lanes to force the enemy team to send someone to deal with it. Banner of Command is sort of a budget Zz'Rot Portal and, while cheaper, is more situational as the promoted minion is only immune to magic damage.
Nami largely doesn't need to purchase offensive items. However, there are a few items that can be beneficial to purchase on her. Ardent Censer gives her a good chunk of AP, CDR, and movement speed, all stats she can utilize fairly effectively. It also further increases her healing values, which is great later on in the game. Zeke's Harbinger is a strong purchase if your ADC goes with a basic crit build, as it will put them at 100% crit when it's fully charged. Athene's Unholy Grail can be a decent purchase if you're ahead and just want to take your healing to silly levels. The CDR and mana regen the item provides are also useful, though Nami doesn't particularly need large amounts of AP to deal damage.
These items are extremely situational, but can be useful in or against certain team compositions. Banshee's Veil is more of a selfish purchase, which I would get against champions such as Syndra or LeBlanc to mitigate as much damage as possible. Frozen Heart can be a good purchase against attack speed dependent champions or against those who utilize attack speed as part of their damage ( Yasuo, Lucian). However, the lack of health in the item itself means you need to itemize health to make the best use of its armor, which can be tricky on a mage support like Nami. Spirit Visage is an okay purchase if you are dealing with a lot of magic damage and simply need a lot of MR to deal with the problem.
Largely Nami's function in lane is to provide sustain, poke, and engage. She also has strong disengage, which makes her extremely flexible. However, Nami, for the most part, will largely go even with or win most lanes if played properly. Mostly you want to poke enemies with Ebb and Flow, use Aqua Prison to engage when enemies are out of position, and Tidecaller's Blessing to either poke with auto attacks or give your carry a buff in a trade. Exhaust should mostly be used in conjunction with Aqua Prison to disengage or to lower an enemy's damage in a trade, though it can be used to chase targets ( Tidecaller's Blessing will usually allow you to forgo this). Tidal Wave can be a powerful engage tool in lane post-six when combined with Aqua Prison, as it can basically guarantee you land the latter when used properly. It can also be used to force enemy champions to back off or to foil a turret dive, as the width of the skill shot makes knocking up grouped targets extremely easy.
Largely your function is the same, however you will often become a key initiator because Tidal Wave is a very strong initiation. You want to use Aqua Prison on high priority targets like their marksman or their AP carry if possible, though you should also use it to peel for your own carries if needed. Largely you'll probably end up using Ebb and Flow on allies and relying on bounces to damage enemies unless they get up in your face, as you're not a front line support. Exhaust should largely be used for peel, chasing shouldn't be a problem if you're using Tidecaller's Blessing on allies who can utilize it well.
Remember that Surging Tides applies to ANY skill that touches an allied champion, this means that Aqua Prison and Tidal Wave will give movement speed to allies. This can be particularly beneficial if you're using Aqua Prison to peel an enemy directly off an ally, as it will give them movement speed while stunning the enemy if placed such that it affects both of them.
Remember that Ebb and Flow deals/heals 15% less damage for every bounce. Because of this, you should make sure to use it on the highest priority target first, whether that is for damage or for healing.