I don't really know but...

Looking at it on a map it looks like it's a huge bay, enclosed on 3 sides. I doubt there would be any significant "swell". There might be small waves whipped up on a windy day but barely enough to trouble you on a jet ski particularly if you're moving.

The main risk would be exposure. Too hot, too cold or too much sun even on a cold day can really fuck you up.

I've sailed there, granted it was on a merchant ship, and it varies a lot. If it's little or no wind or a slight breeze from the west he would be fine weatherwise but when we had a small storm pass through it was a bit rough even on a 200m ship. Would look carefully on the weather report before attempting this, especially since he was towing his fuel.

In fairness, it sounds like my car gets better mileage.

I dunno... Even with a full tank, I bet your car is not going to get very far into the water.