It's honestly really interesting whether people can or cant swim especially historically. In that type of setting if your job doesnt revolve around swimming such as theyre just fishing for fun and its not their job. Its totally reasonable that they wouldnt know how to swim. You'd be surprised how many people cant swim nowadays still. Even in the developed world its not that uncommon to be unable to swim. Even historically lots of sailors couldnt swim and I would argue actually most couldnt.