Originally Posted by SeeThruHead

it's a pretty extreme solution but
https://www.compustar.com/accessories/rps-touch/
would work perfectly for you.

Other things that come to mind are. you could mod your fog light to be a little hide away spot for the key. Some people hide trailer hitches behind theirs so definitely doable.

Edit: NVM pretty sure i have a solution for you

I just tested it. The car wont allow you to lock the key inside. When you try to do it it beeps and wont lock the car. UNLESS, you remove the key from the keyfob and lock the car using the key in the locking cylinder in the door.

Then it has no problem locking the car. And will not unlock if you press the buttons. So just remove your key from the fob when you go swimming. Lock with the key and you should be all good.