Hydron

Hydron is the home planet of The Shoal, native fish people who communicate by flushing their skin a different colour to mean different emotions. The planet is completely ocean except for a few watering holes that stretch vertically from the water and up to the sky. The Ocean Explorer was a submarine that landed on the planet to investigate some special minerals in the water. It was claimed that no one had been on the planet before, but actually Fleming had been there in The Marine Adventurer, which the Doctor soon discovers. (NSA: The Underwater War)