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.
The first question is all about the second one. If you can swim, you may be able to be on the boat with wearing a life jacket constantly (many folks don’t). But, if you can’t swim, they may want you in one the entire time; because, ya know, you’ll float rather than drown.