I never passed the RAF swimming test - I can only manage 25 meters if the pool's that deep.
Managed 26 years flying including long-range (8h 30+) low-level overwater sorties, SAR and MCT without a hitch. Did my hideous 2-yearly dunker runs like a good little boy (except the time I was on penicillin for something or other
which precluded the run). Never saw the point of such repetition - every detail of every run is eternally etched into my memory. It's a drill, like parachuting or spin recovery. Even with STASS, which to my mind is the foulest form of torture ever visited upon aircrew (yes, I've read the other thread on this subject).
So, just 'cos someone can't swim doesn't mean he/she can't do the drills. Personally I'd rather make sure someone spent his time practising the skills required to deal competently with emergencies instead of playing "bob" in the pool. Can he/she get out of the aircraft and into the dinghy with one arm inoperative? Can he/she provide the knowledge and leadership to pax and crew who find themselves taking an early, unforeseen bath in water temperatures of 10 deg or less? Far more useful than 2 lengths crawl/ breaststroke/ backstroke or whatever.