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.
I would not invite someone on a boat I was responsible for if they admitted they could not swim at at least a basic level.
That ability has nothing to do with life jackets