Hydron

Hydron was the home planet of the Shoal, later called the Mermin, 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 Eleventh Doctor soon learned. (PROSE: The Underwater War)

When called to save Professor River Song from a conviction for attempted theft of the Sacred Oar by principal Weeseedra on Hydron, referring to his saving of the Mermin from admiral Icktheus together with the mermin Makron, which had become the legend of the Duktur by then, helped the Eleventh Doctor gain their trust. Taking River as a guilty criminal back to Stormcage gained him more Mermin gratitude. (COMIC: Without a Paddle)