This gives me some story ideas...

For instance, you could go swimming while you're head is safely on land. The main danger there is drowning, which isn't a concern in this case, so you'd have a lot more freedom than usual; so long as it's warm, you could dive as much as you want.

Except, well, you wouldn't be able to see anything and might lose track of your body. Which would be bad.

But if you're a teenager whose head has unaccountably come loose, and you don't think about these things, what chance is there that you'd really think about that? I suspect it would be possible to loose yourself even in an indoor pool, if you can't see what's going on.