Many years ago we were on a beach in south wales and had hired a little inflatable dinghy. I had left my glasses in the bottom to go for a swim and when I tried to climb back into the boat it flipped over and glasses went to the bottom of the sea, about 10 ft down. Bit of a disaster as I was doing fieldwork for a month in S wales and needed them for driving (no spare then).
We waited for the tide to go out and walked about on the beach but still couldn't find them.
A bit later, sitting on the beach we saw some lads shrimping so went to sak them if they might have found them. Just as we got there, one of the lads was trying on what he thought was a pair of sunglasses, and found he couldn't see through them. They were the ones!
So Glenn, maybe a shrimping net is the answer. (Or have a weekend in Thailand and pick up a replica fro £10, - cheaper than a new Tag!)