This is a whale safe break away swivel used on lobster buoys. The swivel has a weak part (highlighted by the blue arrow in the picture below) designed to break away with 600 pounds of pressure.
In the unfortunate instance that a whale might get caught in a lobster pot line, it will break the swivel and swim free. This is another in many responsible steps the lobster industry has adopted (along with escape vents for small lobsters on the traps and biodegradable hog rings which break down and let the vents fall out in the case of lost gear).
The offshore fishery and Pacific coast use 1100 pound break away swivels but our inshore lobstermen use 600 pound ones.
Look for a video with Johnny “Doc” Herrick at 7AM in which we break down the parts of a lobster buoy and their costs.