In order for the sun screen to sufficiently protect against the sun's harmful radiation, it is essential to use the recommended amount:
A bottle with 200 ml of sun screen covers only enough for a single day at the beach for an entire family.
If you apply too little sun screen, the UV protection is much less than the sun protection factor (SPF) promises. For example, if you apply only ¼ of the recommended amount of sun screen with SPF 15, it will only provide protection equivalent to SPF 2.
It is also important to apply sun screen frequently, especially after spending time in the water and if you sweat a lot - even if you use a waterproof sun screen.