AlphaTrigger saidThis is probably not the best site to go to to have peculiar fetishes indulged, OP.
But I'll agree with what davidingeorgia said - the casual wear of speedos for water activities falls on a particular side of a generation gap.
Baby Boomers (50s+) and older seem to enjoy wearing them, which given the physical conditioning of the average Boomer is probably not terribly appealing in most cases.
Guys from Gen X and late Gen Y (currently early 30s - late 40s) skew heavily into the square cut trunks to full cut board shorts.
Millennials and younger (under 30) will rarely found in anything less revealing than knee-length jammer type board shorts for fear of exuding any form of "phaggotry". Unless of course they just happen to be really horny twinks who will be wearing as skimpy a square-cut trunk as their frame can manage.
This all exists in exclusion to any sort of actual competitive water sport where the gear worn is dictated by some governing authority and neither the whims of either age nor a dis-oriented sense of what is age or bodyfat level appropriate.
Alphatrigger pretty much said it all. I'm 43, and grew up in Germany. Until I was in my mid-twenties, speedos was what everyone used for all water sports. When I moved to the US in 1997, I quickly learned that that was not appropriate here, and started wearing board shorts like everyone else.
For surfing, I usually wear compression shorts underneath, and a short-sleeved rash guard if the water isn't cold enough to require a wetsuit. I somehow get a rash on my skin airily easily when laying on the board, and the rash guard also adds UV protection. I do surf shirtless once in a while, usually in the evening when the sun is not as intense anymore.