This boat's weird and a little alien. You might find it beached along some long-forgotten tributary. Being of stone and untarnishing metal, it could have been abandoned for centuries. My players found it in the Underdark, but it'd work just fine outside too.
The boat is made of stone, thirty feet long, ten feet abeam. It has neither oars nor sail. Instead, it’s built around a screw. A capstan turns the screw, which pushes the boat through the water. You can turn the capstan the other way to put the boat in reverse. Though the rest of the boat is stone, the screw, gear, and capstan are all of a light, untarnishing metal unfamiliar to the metallurgy of the surface world. Also unusual is the device on the prow – a corvus, a sort of droppable gangplank with spikes. You ram an enemy boat, drop the corvus (which sticks in the ship) and run down the gangplank to board it.
At your table, the boat itself can function as a mystery. Who built it? Are they still down here? If you have any outstanding mysteries of a similar nature in your Underdark, you can tie it in with them. It’s also a fun setting for nautical combat on a vast sunless sea. My players encountered a subterranean sea serpent, but you could also have fun fighting off Underdark slavers who have ships of their own, flying monsters who dive down from the darkness in the soaring vaults of a lightless cavern, or aquatic humanoids swimming up from below to climb aboard. The corvus isn’t just a boarding tool. With those spikes, it’s a weapon in its own right against some of the larger creatures of the deep. Plus, the thread of the screw is sharp and can be run over aquatic beasties to tear them up.