We should go more by the opinions of GM's, and Nigel Short said:
"Anybody who has seen Garry Kasparov by the swimming pool will know he is extremely hairy"
http://articles.baltimoresun.com/1993-09-03/news/1993246095_1_garry-kasparov-chess-championship-times-chess
This can only be taken as an admission that seeing him by the swimming pool was something quite common, so he should have been swimming a lot.
I think it should be decided on the basis of who was the best swimmer. Fischer has gotta take it. I don't remember seeing any photos of other chess players under water. Tetsuo, do you have my back?
I'm not convinced that photo says anything about Fischer's swimming capacity, and anyway he was just under water 1970-72.