Scarlet Marpenoth Submarine (Patreon)

The Scarlet Marpenoth was a red vehicle shaped like a manta ray.
The submarine had a top speed of 2 mph (3.2 km/h) and required a minimum crew of two in order to be properly piloted and kept. Its interior was always kept dark, so that its crew had to rely on darkvision.
The ship was maintained by a crew of rock gnome engineers and run by a drow crew under Jarlaxle's command.  The ship's air supply circulated between all its chambers through a complex ventilation system powered by quasi-magical machinery. Its airtight doors were kept always locked.
The Scarlet Marpenoth's engine was a quasi-magical device that controlled the ship's depth, as well as the steering fins and rudder, and powered its propulsion system. It could be piloted from the ship's control room.
The ship had a defense system that allowed its outer hull to become electrified for a short period of time.

It is described in details page 141 of Dragon Heist.