Thanks for the replies. I've always had rubbish skin, in as much as I burn quite easily. S yes, I do have top quality waterproof sun cream (and 4* UVA) which I apply liberally. However, it is pleasant to go shirtless for maybe a quick 30 minute swim. I wouldn't normally sunblock myself for 30 minutes, but I do wonder if I will now need to. It's going to be exciting finding out!
Actually, it might be a little too exciting - when we arrive on the island in the Gulf of Thailand, our first excursion will be a snorkelling trip to shark bay, which is full of... sharks! Maybe UV shark repellent would be more appropriate.