Up-voted :)

I like Nami, but I think there are a few things you could work on for your guide:

cool down reduction

You choose Greater Mark of Armor which is a sub-par rune when you consider Greater Seal of Armor provides you with more of an armor buff. The Greater Glyph of Scaling Cool Down Reduction is simply a bad choice. Right now you are sitting at 48% cool down reduction at level 18 and CDR caps at 40%. Which means by the time that this matters you literally won't be able to use it.

Rarely buy list

There are some things I agree with here, but one I cannot agree with. That is Shurelya's Battlesong. It is a great item to engage with especially with Nami after you use her ultimate. I have tested this out in a few games where you have your team together. If you use your ultimate then the speed boost from Shurelya's you can very quickly initiate and destroy the enemy.

I am down-voting right now, but keep in mind I am only down-voting to push you to better you guide. In order to get my +1 you must fix the cooldown reduction. I can not stand for a waste of space (either runes, masteries or items). That waste of space IMO makes the build not viable. PM me if you fix this up and tell me what you think of Shurelya's.

Cheers

Changed my comment, because it was a bit cruel.

You deserve a +1, only thing I find disturbing is your lack of the new items :P

There is a ward item now, that allows warding for much less gold, I really really advice it as a support being.

The guide looks good, for that reason +1
-Meiyjhe