Oh man - can really hear the fear in his voice. I was planning to do something very similar to this but decided against it. My plan was to buy a dummy ring which would fall into the water, I'd then dive in a panic to look for it but have the real thing in a zip pocket, so when I did finally come up I'd have "found" it.
I'd planned the whole thing out but didn't go ahead with any of the execution (calling people up about it, etc). Don't think swimming in Venetian canals (on purpose) was a great idea with all those rats!