The Marine Adventurer

The Marine Adventurer was a submarine that visited Hydron before The Ocean Explorer. All the crew were killed except for Fleming, whose role on the submarine was to collect fish roe. (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 Marine Adventurer, not the Marine Adventurer.