A German submarine
A submarine is a specialized watercraft that can operate underwater, employed by most Earth navies.
Prior and during the Second World War, 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 onboard 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")
Background
The sets of the USS Defiant and the Enterprise NX-01 were designed to be reminiscent of submarines interiors. In the former's case, one episode in particular, DS9: "Starship Down", was intentionally reminiscent of well-known submarine movies such as Das Boot.
A U-Boat appeared in the opening credits of ENT: "In a Mirror, Darkly", indicating this submarine-type was employed on the mirror universe Earth also.