[2.3] Playing like a shark - Nami guide

Let me be very clear - Nami needs mana more then I need Ben&Jerry's after a heartbreak. Her skills might have a short cooldown, but they cost a generous amout of mana as well. As such, taking Sapphire Crystal will allow you to start stacking mana early, allowing you to spam skills more often while also fully stacking Seraph's Embrace quicker. after your first recall, you will have around 1400 gold about 85% of the times. I would typically buy Tear of the Goddess along with Ionian Boots of Lucidity . Having the tear will allow you to spam skills and keep stacking for Archangel's Staff , while Ionian Boots of Lucidity offer more speed and cooldown. Don't worry about stacking AP yet - Nami has a kit that offers utility more then damage, so ap is not something that should be build quickly. Mana stacking is more important early on. 

After that, top your Ionian Boots of Lucidity with a nice Redeeming Enchant . The guide will teach you a cool combo you can do with it later on. Finish bulding Archangel's Staff , as it will offer you a ton of mana and cooldown reduction. It will also safe you in a pinch because of Seraph's Embrace passive, which gives you a shield using your mana. 

After you have those, continue with the three best items for support: Ardent Censer , Harmonic Echo and Athene's Unholy Grail . What you should build first depends: If your team can benefit from attack speed, build Ardent Censer . If the enemy has a poke comp and you have a lot of healing to do, build Harmonic Echo . If the enemy Katarina wants to shove a dagger up your bun, build Athene's Unholy Grail . Regardless, you should build all three of these items.

If you get to late game (and that's a big if) I'd reccommand Rabadon's Deathcap for more AP and either Morellonomicon or Rylai's Crystal Scepter . Remember - more AP = better heals, shields and buffs. Taking Morellonomicon will be useful against teams with lots of heals, while Rylai's Crystal Scepter will cause even Ebb And Flow to slow, triggering Weakness on EVERY one of your skills.