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Just returned from Port A, a couple of weeks ago. I wore nylon shorts with belt loops, that would work for swimming. Since I was not going into the salt water while carrying, I used an alien gear IWB, holster for my Ruger Security 9 compact. I wore a nylon belt with a plastic buckle, and was very comfortable.
I was fortunate, that when I did decide to go swimming, I put my gun/holster, in a beach bag Mrs Jusme brought, and it was left under the big tent, surrounded by trustworthy relatives. The rest of the time, it was on me, while walking the beach, driving our rented golf cart, eating out, etc. I cleaned and oiled it every evening, and had no issues.

You may want to look into a locking gun safe, that can be secured inside the vehicle, with a cable around the seat base, while swimming, or otherwise away from your firearm. JMHO