Hi Emca, Interlaken isn't such a good place for water sports. The water is cold, there are many boats, and the lake sides are very steep.
Have you considered Lake Biel? The water is warmer, it's not so busy, and easy to get to the water. There is a small road from Twann to Tüscherz along the lake.
Note, you need to start at Twann, as the railway is not easy to cross over. https://map.search.ch/Twann,Bahnhof....16,217080&z=64