Firstly, let’s clear up a big misconception with swim caps: they are not designed to keep your hair dry! I know this may come as a shock but no swim cap is guaranteed to keep hair dry. Although some caps will do quite a good job, this is not their purpose. To try to keep your hair dry as much as possible you can either wear 2 caps (maybe a Lycra cap and a silicone cap over the top) or use a silicone cap which gives a tight fit and should keep most of the water out.