As a former Surfer (before hip replacement!) I can tell you the ol' Surfer's Trick: Never keep anything in your bathing trunks or wear jewelry or a watch. Hide those objects (including keys) somewhere onshore, in your car, or under a dandelion or a rock, some secret stash. Don't let anyone watch. Have fun + retrieve your treasures.
During extreme low tides, I used to join others and walk the rocks at Malibu, Topanga, and other rocky points. I would find colorful surfboard fins, rings (some silver), pendants, watches (usually rotted out - so much for Timex Waterproof Timepieces), all sorts of stuff. All having fallen from luckless swimmers and surfers who went into the waves adorned with valuables. Well, the fins came from beneath surfboards of course.
Best find ever: a barnacle-encrusted dog collar with the owner's phone number on it. I called and asked if his dog had drowned. (I am soooo tactful, sometimes.) It hadn't; they had gone swimming somewhere near Zuma Beach and the collar slipped off and drifted southward, ending up miles away in Malibu about six months later. Happy owner came to my house and retrieved it.