- You may be looking for the Earth sea vessel.
The Fortitude was a Voord vessel on the aquatic planet Hydra. It was the head of a military flotilla. Fortitude was equipped for travelling across water, and also had a sonar for tracking nearby submersibles.
When the TARDIS landed on the cargo ship Hopeful, the First Doctor, Ian, Barbara and Susan were discovered by the crew and taken to the Fortitude. The ship was attacked and sunk by the Voord with the Doctor and Barbara still on board. They managed to survive, but were soon captured by the Voord. (AUDIO: Domain of the Voord)