The best sunscreen will be a full boddy suit, like the one used in Queensland for stingers protection and a balaclava, with holes around the eyes and for the mouth. Sunglases on top of the balaclava are a must.
The advantage of this, is that you'l scare all other beach goers away, you'l have the beach all for you, and you may get a free ride in the police car

The best sunscreen, is the one you use and help you: must be at least 30+, the thicker they are, the best chance to last at least two hours, and don't use it to close to the eyes: all, but all of them sting, and it is not fun to kitesurf with watery eyes

Cheers