World War II - USS Blenny SS-324 - 312 ft, 1,810 tons, 8 knots submerged
Everyone knows what a submarine is - a ship that can go underwater. Hardly needs any explanation. Unless you're in the US Navy. By the Navy definition, only their modern, nuclear-powered boats ( all submarines are called "boats" ) are true submarines, designed to operate beneath the surface almost indefinitely. Anything else is merely a "submersible", tied to the surface by the need for fuel and air. Whatever.
This page lumps together representatives from crustaceans and mollusks. Freshwater typically has lower diversity than the ocean. However, this is just a tiny sample of what may be found.