How to counter Nami
Let us take a look on how to beat Nami. We will look at champions who are strong against Nami and who are weak against Nami. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Support champion who counters Nami. You can already win in champion select with the right counters for Nami. If you want to know how to counter Nami then you need to take into account the best picks vs Nami and the worst picks vs Nami. This Nami counter data is based on matches played by Pros on patch 13.24 and previous ones. It is important to see whether Pros counter picked Nami or whether they played as Nami.
Champions used by Pros to counter pick Nami
Looking at the matches from Pros (see table Pros counter picked Nami) we can see that they picked 
Senna most often to counter pick Nami. The matchup Senna vs Nami occured 10 times. Nami has 5 wins and 5 losses in this matchup. Learn how to counter pick Senna to get another perspective on the data.
Lane outcome for Nami
When looking at Nami counters you will notice that win rates alone are not conclusional. Therefore we provide an overview (see table Lane Outcome) where you can see the lane outcome of Pros who played Nami. The lane outcome of Nami consists of three outcomes: Win, Loss or Draw. Let's take a look of Nami lane and match outcome for specific matchups. Considering the lane outcome and the match outcome of a specific matchup, might enable you to find the right counter for Nami.
Conclusion
Analyze the available data and learn how to beat Nami. To master Nami take also a look on Nami builds and runes for Nami. Good luck, Summoner!