The Ocean Explorer

The Ocean Explorer was a submarine used to explore the water on the planet Hydron.

It was captained by Jane Clancy, but taken over by Fleming. It was used to first collect fish roe, which Fleming had gathered from within a mountain, then to travel up the caves in one of the mountains to return them to the water and back to the Shoal where they belonged. (PROSE:  The Underwater War)

Behind the scenes

 * Unusually, the definite article the is actually a part of the sub's name. According to the text of the book, it's The Ocean Explorer, not the Ocean Explorer.