It means that after 20 minutes in the water the sunscreen retains 50% of its effectiveness (labelled SPF). Some sunscreens are labelled very water resistant, which means that they retain 50% of their effectiveness after 40 minutes. Given this, you can see it’s essential to reapply afterwards. The very water resistant products tend to be less cosmetically acceptable – very sticky – and thus not the ones to use daily, but are probably the best to use if spending long periods of time in the water e.g. a long session of surfing.
It’s not correct to say that a cream is water proof.
