I strongly recommend that you look at a copy of Aqua Erotica, to get a sense of what type of stories worked well the first time around. As in the first volume, the only ground rule is that the stories must address desire, sexuality, and, in some form, water. Travellers meet strangers, in cities where no one knows their name.
Lovers cool each others' sweaty bodies with wet kisses, with water, with ice.
Summer is about heat, and so it is also, inevitably, about the need for water. Other stories might take liquidity as a way to explore the transformation of identity in erotic passion. Others might look at the surreal or fantastic. If you want to use a swimming pool, use it imaginatively. Tell me a story about water and the body, about sex and wetness, about liquidity.