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
That’s what socializing on the boat, dockside is all about. Some of the fun, little to none of the danger.