I like Proderm products. They come as a mousse (easy to apply)and claim to require only one application every 6 hours. Manufacturers also say products are waterproof and sweatproof and even light-towel-drying proof.
I wore the stuff in Sardinia, snorkelling - think exposed pink British back - and never came close to sunburn.
Trouble is, its hard to find. Sometimes see it in Tesco, Sainsbury, Superdrug, but they usually stock it only for a short time then whip it off the shelves again.
Anyway, dark blue can with orange top, if you want to check it out.