Most starships that have a “Cetacean Ops” segment carry either https://en.wikipedia.org/wiki/Bottlenose_dolphin or https://en.wikipedia.org/wiki/Beluga_whale (actually a very large dolphin). Both of these species require a marine environment and eat an enormous amount, which would make them very uncomfortable on a small ship like the Trouble. Instead, Hiuw and Fred are https://en.wikipedia.org/wiki/La_Plata_dolphin, a much smaller species which is at home in both rivers and estuaries, and is far more able to handle variations in temperature and salinity than either of the usual cetaceans.