Hydron

Hydron was the home planet of the Shoal, later called the Mermin, a race of fish people who communicated by flushing their skins with different colours to express different emotions. The planet was completely covered by ocean, except for a few watering holes that stretched vertically from the water to the sky.

The Ocean Explorer was a submarine, landed on the planet by the Cosmic Rover, 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)