Most gay men are into this niche and don't know it.

When you are at a pool or the beach and you see a guy in the water. You keep tabs so when he gets out you can see the "cling" of his clothing against his manhood.

It's a major cross niche from or into - spandex, wet and messy, tight clothing, mud play, wet suits, workout/sweat and piss or wet jeans.