Awhile ago I read a Yaoi where the uke went to the beach on holiday and was very timid. He loved to swim but burned easily so was always under an umbrella. He always saw the seme there surrounded by girls. The seme was tan and very strong. While swimming the uke almost drowns due to a cramp and is saved by the seme. Afterwards the uke pretended to not know how to swim. The seme starts 'teaching' the uke to swim and they wind up having sex on the beach. Does anyone know the Yaoi? I know it's not Sailor Danshi.

Kowarekake No Omocha Chapter 5 has a very similar storyline, but it's probably not the one you are looking for since the uke doesn't get terrible burns...