I'm glad to see you've updated this recently. Keep it up, I really like this guide! Ap nami has always interested me due to the special ap scaling on her w.

Good idea but needs some more details! >///<

Hiya~
I see where you're going with this but it's no debate that investing in ability power marks and seals is really, really bad.
I suggest Mpen marks and armor seals (since you would have hp from roa anyway, but health per level seals are also nice)
Magic penetration is hard to come by in the game, unlike ability power which is present in almost all Mage builds. The flat Mpen is also pretty good to start with and should really be picked over the meager 5.31 ability power you will get by investing in 9 ability power marks.
Same should be said for seals, 5.31 ap can't amount to 9 armor or the defense you will get from health per level seals.
Runes aside, you could definitely work on improving this guide a little more.
Match-ups, tips, playstyle, etc.
I'll reserve my vote until further amendments. :)