Do you know anyone who is a nurse or works at a health centre? When I did events in France they're mostly concerned with the official stamp & mine certainly weren't the result of any medical examination.
When I did Vichy there was a doc around to 'upgrade' mine as I only had a cycling letter but now that they've moved to M dot not Challenge, they might not offer that service.
Unless it's changed in the last few years then the swim isn't that bad. I'm also a slow swimmer (most pirates are) so I was near the back & sighting was OK with only a slight bit of chop after the turn.
The water is a bit murky but the event has been running for a while now so I don't think there's too much to worry about in terms of water quality.
Quick tip for the swimming - swim as if you're holding a coin between your cheeks. Keep that clenched & it helps you to keep straight legs when kicking. Takes a bit of practice but getting a more streamlined position can improve your efficiency and stop you wasting too much energy.