When I went on my honeymoon in '98, i brought my detector. 1st day there, the wife and I went on a snorkel trip. I jumped into the water with my hand in a fist to keep my ring on. I already told my wife to leave her rings in the room.
As soon as my head broke the surface of the water, I heard "F@#$", and I looked down. the couple that jumped in right after me BOTH lost their rings. Where the boat was anchored was about 70', with a short wall up to where the "easy" snorkelling was. Myself and the divemaster made several attempts to get down to the bottom, but we couldn't find the rings... they buried themselves real quick in the sand.
I told the couple... no big deal, I'll just bring my detector back on the next trip, I'm sure they won't have a problem with it. So we finished the snorkelling trip, went back to the resort, and I grabbed my dive gear and surfmaster pi... and back out I went. We arrived, and I hopped into the water, and went directly down to where the rings fell. The divemaster from the first trip was with me, and after about 20 minutes, I had 6... count em... SIX rings in my BC, not counting a couple bracelets, a really nice ROLEX dive watch and some assorted change.
We had some time left, so we swam over under one of the other boats from a different resort, and did a search under that one too. We found a couple rings and some change. When we got back to my resort (Sandals, St; Lucia), I went to the couple's room, and they picked out their rings. then I went to the front desk and said I had found some jewelery, and if anyone lost anything at the dive site, to please contact me.
Later that week, I returned 1 of the rings and that ROLEX (damn). and over the course of the week, I got called twice to the volleyball pit to find rings. I came home with a good haul, and several bottles of REALLY nice RUM from some of the other guests.
DEFINITELY take the detector... especially to resort spots.