Us lot use a number of options:
Keysafe’s (little coded locks with flap for keys) deterrent at best, easily opened with a screwdriver unfortunately, but it’s the better option because it’s seen on the car.
Aquapack’s, sealed leak proof bags with lanyard for your neck. Nice enough by the lanyard can throttle the life out of you if not careful… under wetsuits not so bad because the neoprene presses against your body trapping the bag to your body, but they do work thier way down to the weight of the bag/key. We hang em’ around our necks and throw the key back over our backs so not to interfere if we land on our chests climbing on/off boards etc.
Condoms, useless. They break/rip and you loose the key to the water ingress.
Sandwichbags, fine for short periods, tend to leak if not fully wrapped and maybe taped up.
Hiding key under/on top of wheel or under bumper.. well, can be ok but the wheels the first place anyone looks on the beach, the bumper recesses are ok if you are lucky, alloy wheel gaps are useless as wind gets through and key can get blown away/lost in sand.
Hiding key somewhere on beach, effective if you can remember where it is, but kids and detectorists scour the local looking for buried treasure..and hey presto’ they’ve found a key!
Swallowing it.. not an option unless happy to wait until morning for the inevitable..
And the whole proximity key fiasco is proving absolutely hilarious these days for these keyless entry fangled dangled modern solution to a problem that never existed..
HtH’s