Exercise before food?

Exercise (but don't drown) anytime you like, it's all good. Try using your meter to prove your theory. I have never heard of it before but that does not mean it's wrong.

If it was me, I'd be worried about the lack of lifeguards. Is it a dangerous bit of coast?

I used to swim for exercise, but always did it after food, usually after my evening meal. I also ate some chocolate first as I found swimming used up lots of energy. I imagine sea swimming could use even more.

If this is a new routine for you, could you perhaps try something like walking or running first to see how your BS is affected? Then you could decide better about an early swim with that info.

Hiya, welcome. I too live by the sea. I would advise caution here. Not because you are prediabetic (thousands of people statistically are and don't know it) but unless you know your coastline and the currents, sea swimming can turn lethal. As a supporter of the RNLI I'd wait till 10am and make sure the lifeguard sees you go into the water.

OK, then I'd say you're good to go! I do admire hardy swimmers. I freeze looking at the sea
If you happen to test your BG before and after, let us know.

The problem with exercise is that any strenuous exercise will drop your blood glucose (bg) levels to the extent that your muscles and liver will release their stores of glycogen (stored glucose) and your bg levels will rise due to the injection of glucose then if you eat after exercise that can push your levels higher still, better to exercise gently after eating as that will tend to lower your bg levels.

Gentle exercise generally lowers bg levels
Strenuous exercise will generally raise bg levels

Bear this in mind when planning exercise and check your levels frequently and you will soon learn what lowers and what raises your bg levels.

I was a massive swimmer at one time, but was always told, and believed, we should never swim on a full tummy for fear of cramp.
Old wives' tale? No idea.

By the way, exercise does not necessarily bring your levels down immediately, or at least it doesn't with me. I find my levels increase after exercise for possibly up to 40 or 50 minutes later. If you swim in the sea after an overnight fast, you are likely to find you get a large liver dump. Your liver will be under the impression your body is short of glucose and try to look after you by dumping glucose. Best to exercise after eating, an hour or so after.

I used to do my horses as soon as I got up and then tested when I came in to have breakfast. My glucose was always high. I then started testing before and after doing the horses (exercise) and found that my BG was between 1.5 and 2,3mmol/L higher after doing the horses. I started testing, having breakfast and then testing again I hour into doing the horses and two hours after eating and found my glucose was much more stable.