I like your theory about England not being able to swim, though I always thought that it was because in the past it was considered unlucky for a sailor to be able to swim (the idea being that you couldn’t cheat the sea).

There’s plenty of other reasons for this, but I like yours too!

Some others would be the fact that there weren’t lifeboats, most of the sailors were urbaners and didn’t have access to bodies of water, or even the belief that water held diseases and would kill you either way.