If you want to take someone sailing on a boat and you ask them if they can swim, what exactly are you asking? Everyone can float if they have a life jacket right?
My understanding is that someone is being asked if they can take care of themselves and/or tread water until they are rescued and/or if they are a strong swimmer, but I could be wrong.
Yeah, plus being able to swim means being able to maneuver with the lifejacket on in addition to basic water safety knowledge. Being able to maneuver a short distance with a life jacket makes getting back on the boat s lot easier.