In order to legally classify a sun screen as water-resistant, half of the original protection still works after 20 minutes of swimming twice and 15 minutes drying twice without using a towel.
For a sun screen to be considered very water-resistant, it must be applied in two phases: 20 minutes before going into the water and 15 minutes after the skin is dry. After which the SPF will still be at least 50% of the initial SPF.
All Derma sun screens are classified as very water-resistant. Important: Always apply sun screen again if you have spent time in the water, perspired or dried using a towel, to be sure that you are optimally protected.