Nami's kit has both a slow Q and R, so she is pretty hard to be aggressive with against good opponents. What she excels at is punishing bad plays the opponents make such is being too close or engaging. People play her aggressively, but I think she fits more into a Janna type of Support, where you can reset fights or cancel the opponent's initiations.
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I can carry with her lol, her healing fish kill steals, her Ult can win teamfights her stun can nab multiple enemies not just one she's a beastly support and if ppl don't know that then they are in for a surprise.
You should try her the next time shes free I started playing her a month ago roughly and she has quickly become my favorite champion since pre rework Sivir.
The main problem is landing her Q, I used to play a lot of Brand and Cho so that helped me but just keep practicing with it and leading people. During team-fights it is amazing for pealing for your ad or if you see the other team clump up you can usually get at least two of them. And until you get the timeing down you can use your ult and e to slow them down which makes it almost harder to dodge unless they use an escape.
Also the passive is not amazing but it is definitely useful especially with the ult remember it works with any of Nami's spells not just the target-able ones there have been a couple times where i've seen my mid running down river and being chased by the other team, I will e myself and then launch my ult towards them, both times they had a dot on mid and I wouldn't have been able to heal them in time without that speed boost.