Not sure on the jock v. thong.
Water polo players like brief style cut for a couple reasons. First less suit to get grabbed and because the fit around the legs on a brief is a much tighter grip than on a square cut, briefs are harder to grab. Second is that briefs do not bunch, but with all the kicking and flipping, square cuts can bunch a bit on the legs, again easier to grab and more restrictive.
For divers, it is alot of the same. Less constriction in a brief and a brief shows off your form better. Diving is skill, but watching a dive is grace. When you come out of a dive, a brief is still tight and smooth on the body, but a square cut might have bunching or creases. Plus the fit of a brief just feels more secure. Having said that Diver Matthew Mitchem does some of his dives in Funky Trunks, which are both brief and more square.
I am sure there are other reasons, too.
Finally, as a swimmer, race briefs just look so hot
