You can buy waterproof cases at Bed, Bath and Beyond and elsewhere I’m sure. Who knows if they work, what they need to do is make dexcom waterproof. I just ruined mine recently by wearing it into a pool. Fortunately I had it on home owner’s insurance so it was covered and quickly replaced.
I’ve used it on several cruises and beach trips. I’ve attached a small clip to it and it’s served me very well over the years. I think I picked mine up at LL Bean, but it looks like it’s available at multiple outdoor stores.