I don't know about swimming pools in your country, but in the Netherlands they all have lockers you can use for your wallet and phone and such. I sometimes use those in busy pools whith people everywhere.
From experience, I know I'll start to drop after about half an hour (may be very different for you!), so that's when I usually check the first time.
At that point I sometimes take a few bites from my apple, if I see the start of a drop, but nothing close to hypo yet. If I see a sharp drop I might add a sweet as well.
If I'm in the pool longer than about 45 minutes I'll test more often, as a drop will definitely come.
More often than using a locker I just put phone or libre reader (in waterproof cover), meter, apple and sweets at the life guard's station, the first aid office or the place where you can buy coffee and chips, depending on which spot is closest to where I am. Yes, it can be stolen of course, anything can be stolen.
If I'm simply swimming laps at a quiet hour I tend to leave my phone and apple on the side of the pool. That way I can scan and take a couple of bites if necessary without even leaving the water. In my local pool they're completely used to me munching on my apple while swimming, and I love not having to take a break because of diabetes.
In the rare cases I drop hard enough to warrant sweets I do go out of the water, I don't want to be in if I'm below 4.