I generally bought it to make a carry carry harder. If I want CDR and health as a support, I'll get Shurelia's, since lifesteal is a wasted stat here.
In any case, someone purporting to have Nami's loadout has posted:
Note these are subject to change :
Passive : When Nami's spells hit allied champions they get increased movement speed for a short duration.
"Hydro Blast" Q : Nami launches a giant bubble into the air, when it lands it encases all targets hit into bubles for a short duration.
"Surging Tides" W : Unleashes a tide of water that surges between allied and enemy champions, healing allies and damaging enemies.
"Aqueous Empowerment" E : Empowers a champion for a short duration, causing them to deal increased damage and slow their target.
"Tidal Wave" R : Summons a massive tidal wave that moves across an area and knocking up targets briefly and massively slowing themAll deal damage. W bounces up to 3 times, always alternating between ally and enemy champion.
Q does 75 to 295 + 75% AP on a 16 - 10 sec CD
W does 50 to 210 + 50% AP on enemies (50 to 170 + 30% for heal on allies) on a 9 sec CD
E can make your next 3 attacks do 25 to 85 + 20% AP each (assuming it can be cast on self, 5 second window to use them) on a 11 sec CD
R does 150 to 350 + 80% AP on a 150 - 110 sec CD.