This is the build I've been using since I began playing Fizz. I focus on Attack speed and Ability power to maximize the power of Seastone Trident and keep as much damage on my targets as possible. This helps get last hits and allows for assists in situations where you have to run.

Between Fizz's passive and Playful/Trickster, defense hasn't been an issue for me, so I focused all my runes on offense, further enhancing what I itemize toward.

Letting my enemies think that Magic Resist will save them is just fine. If they start grabbing MR items I can easily switch my items to boost AD instead and I already have armor pen this way. Cooldown reduction isn't really necessary unless you're trying for a tanky build, as the only cooldown that is bothersome is Playful/Trickster, and Magic Pen isn't a big deal until later, when you'll have Void Staff. The lifesteal and AP/Level are quite nice, and the final mastery in the Offense tree combos well with Seastone Trident.

Attack speed and Ability power early allow for quick kills with Seastone Trident and some decent damage on Urchin Strike. Malady gives extra damage that isn't stopped by armor and lowers their magic resist for the DoT and for your allies' abilities.

Phage into Frozen Mallet takes advantage of Urchin Strike's on-hit capability and gives some survivability in the form of health, some damage, and a convenient slow effect so chasing shouldn't really be a problem most of the time.

From there, Sheen gives some nice burst and builds into Trinity Force to keep them from being able to itemize too heavily toward Armor or MR, not to mention extra health and mana, crit chance and move speed.

After that it really doesn't matter too much, but AP will enhance your Shark and Magic Pen will keep your DoT and your Shark from losing power.

I get Urchin Strike first for a little burst and chase ability, which I sometimes don't need, but level 2 and 3 come fast enough that you could probably pick anything. I max Seastone Trident first because it offers tons of damage in its passive and a great active for turning the tides in 1v1.