Play her with an aggressive team. Qing someone then popping W and E on your ADC is amazing in lane, and her ult wrecks if you use it to follow your team in, especially if you pop Shurelia's along with it..
I have been playing support-Nami almost exclusively the past two weeks. Her Q's are a little slower than Cho'Gath's ruptures, but if you can land them: you're golden. Timing is very important when playing her. If you can balance out the timing of your Q's, when to appropriately poke/heal your ADC, and use her ult during team fights... you should be completely fine.
I can't tell you how many times I've saved my ADC/teammates from being wrecked by Q+ulting the enemy team.
Admittedly, she makes playing Sona so boring. I agree with the early suggestion(s) about having an aggressive team. My ADC is generally too aggressive, but Nami can hold her own (as long as you and ADC are aware of the obnoxious CD on her skills).
Hi, I'm Jae. I still prowl around here. I quit most games, but hi: MS | LoL | D3 | HoN