Hydron

Hydron was the home planet of The Shoal, fish people who communicated by flushing their skins different colours for different emotions. The planet was completely ocean except for a few watering holes that stretched vertically from the water to the sky. The Ocean Explorer was a submarine that landed on the planet to investigate 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 learned. (NSA: The Underwater War)