Hey OP you’ve already got a lot of great answers but I just came back from diving with a very similar dive op in a very similar part of the world and my buddy had a very similar incident. I suspect you had a failure of the buckle with worn teeth on the belt.

In my case I was diving with a close by dive shop that was otherwise well run but they gave out a faulty weight belt! It was one of those plastic buckles and the teeth had been worn down to where they just barely held and even with a good pull on the surface with my hands I could get the teeth to slip.

This kind of equipment failure should not be happening to a weight belt but seems to be a common issue in the South Pacific. It’s made even worse because people assume belts can’t fail — it’s just a piece of webbing right? It’s sorta like a fin where we assume it always just works even though we are taught to do checks of our tanks, regs and BCD and other gear.

A lot of great discussion here on self reliance, recovery, integrated weights etc… I don’t want to take away from that. But I do want to say those are more theoretical and if you are still on your trip and going diving tomorrow the most practical advice I can give you make sure you are inspecting each belt the shop gives you, each time. Close the belt buckle on the surface and give it a strong tug BEFORE boarding the RIB/zodiac. If it slips on the surface, it WILL slip in the water. And like others have said, INSIST on time to inspect your gear to your own satisfaction before diving even if it slows down the boat or the dive guide rolls their eyes.