Same here in Maine for lobsters. Lots of bricks out on the bottom from the ballast for the lobster pots.When I first started diving 34 years ago, I was perplexed by the number of house bricks I found well away from the shore. Finally I figured it out, people were putting bricks inside fish/carb/cray traps as ballast and these were lost ones, the wire cage having rusted away. Perhaps something like that?