The water-filled section of some Starfleet vessels where cetacean officers live and work on navigation, seen aboard the Cerritos in Lower Decks and Voyager-A in Prodigy.
Cetacean Ops is the department aboard certain Federation starships that houses dolphins and whales serving as navigational specialists. The idea began as a background detail in technical material for The Next Generation and was heard only in passing references for decades.
Lower Decks made a running joke of it before finally showing the facility in the second-season finale First First Contact. The Cerritos has to shed its outer hull by hand, and the last release can only be reached by swimming through the flooded compartment, guided by its two beluga officers, Kimolu and Matt.
Prodigy's second season shows a larger version aboard the USS Voyager-A, home to a humpback whale named Gillian. In Observer's Paradox she helps Rok-Tahk, and the name recalls the marine biologist of Star Trek IV.