It wouldn’t be a sea turtle imo because they eat sponges, jellyfish, algae, seagresses and corals depending on the species and don’t eat fast moving fish where they need that snakelike head.
But....Florida Softshell turtles get pretty large, have the snakelike neck as they eat fish and do venture out into brackish water on occasion (where if they die they could get swept out into the ocean for a time). And sharks or other predators might take a big bite out of the softshell especially since it’s basically just skin.
So you may be on to something....
The Florida softshell in question