Although L. Frank Baum wrote a fantasy taking place under the sea and referenced it a few times in the Oz books, he really didn’t say that much about aquatic life in Oz and its surrounding nations. In Glinda of Oz, Baum tells us, “In Oz, where all the animals and birds can talk, many fishes are able to talk also, but usually they are more stupid than birds and animals because they think slowly and haven’t much to talk about.” Perhaps this is an attempt to justify why no one really seems to have a problem with fishermen, when you’d think the animals they caught would be sentient. Then again, we also see hunters on occasion, and we know Ozian land animals can talk. It’s a question that’s never really resolved, but it does seem like fish and caught and eaten quite often in the series. Remember the Lazy Quadling’s wife catching an eel, for instance? One of the nasty deeds for which the lonely ferryman in Lost Princess is punished is taking a fish out of the water and leaving it on land to die, although this might not have been as much of an issue if he’d eaten it instead of torturing it. Some fish do play an important role in Glinda, but these fish are actually enchanted humans.
As far as fish and other aquatic creatures in Oz go, we don’t come across all that many unusual types in the series. In Emerald City, the Tin Woodman has a pond containing tin fishes. In addition to the red eels, Patchwork Girl informs us that the Trick River is inhabited by giant fish strong enough to overcome its constantly changing current.
When Dorothy examines the lake from the submerged Skeezer Isle, she sees starfish, lobsters, and crabs. Ruth Plumly Thompson introduces the A-B-Sea Serpent and Rattlesnake, visitors from the ocean, in her Royal Book. In Hungry Tiger, a fisherman finds one of the lost Rash Rubies inside a fish, something that happens a lot in fairy tales. Giant Horse brings us the monster Quiberon, large sea horses that can be ridden (Quiberon eats them, but the Wizard of Oz is able to restore them from their bones), and an old merman. While this merman lives in a salt lake, there are mer-folk in fresh water within Oz as well, at least according to Sherwood Smith’s books. Lady Water Lily from Enchanted Island describes herself as “a kind of mermaid,” even though she has legs instead of fins.
Thompson calls her “the Lady of the Lake,” so maybe she’s the same sort of fairy as the one who gave Excalibur to King Arthur. In Lucky Bucky, Lake Quad is home to an Octopuss, a sort of eight-legged cat that lives underwater.
According to Glenn Ingersoll and Eric Shanower‘s Trot, this lake also has its own Nessie-like monster, Quaddle, although he only emerged from a lake-quake that buried him for eons shortly before the story began.
Eric brings aquatic life into his Secret Island as well, with the impetus for the story being that the Royal Gardener has every kind of fish in Oz in the garden pool except for the Crimson-Tailed Quipperug, which Dorothy and the Scarecrow set out to acquire. There’s only one known Quipperug in Oz, though, and it doesn’t want to relocate to the Emerald City. During their quest, they are assisted by a large fish that can take air-breathers underwater in a bubble. So if the garden pool contains every sort of fish in Oz except the Quipperug, does it have one of these bubble-fish, or is that a species unknown to the Gardener? It almost certainly doesn’t include saltwater fish, despite the fact that Oz has at least two bodies of salt water, Lake Orizon and the Inland Sea from Lost King and Enchanted Island. In Bill Campbell and Irwin Terry’s Masquerade, the Patchwork Girl accidentally stumbles into the aquarium in the palace conservatory, which is home to many fish, a small octopus, and several ancient turtles with emerald-encrusted shells. According to a turtle’s testimony, the aquarium also contains fresh water.
The book in the series that does the most exploration of underwater life is Captain Salt, which takes place on the Nonestic Ocean. In this book, the captain and his crew discover monkey-fish, flying fish with wings and claws, sea lions with scaly manes and webbed feet, a bizarre sea serpent, a quite large narwhal, and a society of jellyfish people. These people have “round, square and triangular faces” and live in a place called Seeweegia.
They capture Captain Salt to put him on display, but Tandy rescues him and he kidnaps a jellyfish boy. This is rather disturbing, as the jellyfish people are intelligent enough to write in letters that the crew can read. Roger also mentions having silver fish at home on Octagon Isle, presumably meaning a kind of fish rather than the insects of that name. You wouldn’t think a Read Bird would be too fond of creatures that eat books, after all. Flying fish that actually fly show up in Ozmapolitan as well, although these are bigger and are specifically said to have feathery wings. They live in the caverns on the Winkie River.
I’m sure there are many other creatures in the lakes and rivers of Oz and the Nonestic Ocean that have yet to be discovered.