One way to deal with a good Nami is to go for the Nami instead of the ADC. Zenith+Stun Nami and Exhaust the enemy ADC and you should survive and win the trade/get a kill if your ADC follows up. After 6 Nami should prolly die in Zenith+Q stun + Ult stun combo before she can even do anything.
But as you said, it's mostly about the players, not the champions. Though champions counters and whatnot do play a pivotal role when the skill is roughly equal!
Bottom line is, I just really like Leona <3