You will play hobb trying to retrieve them with a magnet! Back before 1965, the city of New Orleans used to dispose of unwanted and seized handguns by throwing them in the lake.
Your will be just one of many rusted beyond use.
My granddad must've found one of the NOPD dumping spots by accident. He pulled up a crab net and snagged a nickel-plated Colt New Pocket revolver. It must've have been in the lake for very long, because an oil bath and scrub showed it to be in good shape, all things considered. My cousin still has it. Even though a gunsmith has checked it out for cylinder timing, etc., he's afraid to shoot it.