It does sound a bit much, but in 1976 I was SCUBA diving not more than 20 miles from my house, when I found a Mastodon tusk in about 30’ of salt water.
Off the coast of FL. somewhere? Details please. I suspect is was washed into the salt water area recently, or came from a recently eroded shoreline that has been spilling artifacts into the water for several years. By now there may be a whole Mastadon skeleton laying about. I'd go back. Florida's springs are full of the bones of Mega-Fauna from the last Ice Age, as you well know.