A German submarine
A submarine is a specialized watercraft that can operate underwater, employed by most Earth navies.
Prior and during the World War II, Germany was one of the world's leading nations in the area of submarine technology. Their submarines were named "U-Boats", the term deriving from the German Navy's system of naming its submarines with U- followed by a number, where the "U" stood for Unterseeboot (literally, "undersea boat"), the German word for submarine.
Malcolm Reed's great-uncle once served on a submarine in the Royal Navy. Reed's uncle died while trying to stop on-board flooding after the submarine struck a mine. (ENT: "Minefield")
In the Julian Bashir, Secret Agent holoprogram, a submarine was waiting for the secret agent near Iceland. (DS9: "Our Man Bashir")
Submarines also existed in the mirror universe. Among them were U-Boats and Project 705. (ENT: "In a Mirror, Darkly")