Undersea Kingdom Categorization.

1. Formerly a land kingdom but just look what happened.
2. Currently operates like land because of magic, divine providence, supertechnology, etc.
3. Sea creatures.

Those are the categories I can think of. Do they cover everything? Watery aliens are three, ghosts and undead are one, the future is two. Atlantis one, Jabberjaw two, Snorks three. Yeah, I think that just about does it, but I'm open to correction.

Addendum

Some might protest that making convicts live inside a whale is essentially a death sentence. Those people go in the whale.

Incarceration Solutions

Prisoners often end up inside whales, but fantasy governments should consider sending them there directly. It's like exile except nobody gets mad at you for sending your worst junk.

The World of Prophecy Is Limitless

The oracle declared, "A whale and a lion will fight in the middle of the city!"
The wisest debated the hidden significance, whether it referred to a debate over strengthening the army or the navy for example, until one day a whale and a lion fought in the middle of the city.
"But now we need a reverse oracle to explain why that happened," one of the prominent citizens said, but in the meantime the people consulted the oracle again.
"A whale and a lion will fight in the middle of the city!"
"The same ones?"
To that the oracle had no answer. The citizens therefore waited, and they learned when next the event occurred that it was a different lion, though about the whale they were unsure. The original lion returned for the third match however.
The citizens became so used to tthat state of affairs that they were unprepared when new mines were discovered and a debate began over strengthening the army or the navy, but that struck them as trivial compared to the other matter. In the end, they relied on the lions and whales to dissuade invaders and spent the money on a new racecourse.
Finis

Addendum

"Tidal fantasy" refers to how the tides have shifted in various ways, but there will definitely be giant whales that eat whole ships in it.

Somebody Gets Eaten by a [ANIMAL] and There Are People and Such Inside

Traditionally it's a whale, going back to ya boy Lucian, but don't forget about elephants, rocs, dragons, and gods. That last one kind of happens in the Mahabharata, but that doesn't mean we can't rip it off. I like the elephant one the most aesthetically, but rocs have the most story convenience. Because of the travel, you see.

The Difficulties Are Perhaps Insuperable

A pebble bounced down the hillside and straight off the island. Doanu watched its progress while Ferdin stared at the ships entering and leaving port. "You know, Doanu."
"Doubt it." He kicked another pebble.
"I bet we could apply some of the construction principles of our ships to build ships suitable for other environments. Obviously we'd have to make all kinds of adjustments, but I really think it could be done."
"'Other environments?' Come on. What are you talking about? I shouldn't ask. I know you have no idea either."
"I'm talking about seaships, lakeships, riverships. All kinds of ships. Waterships!"
"Uh huh. Why would we want those?" Doanu swept his hand as if to mark all the sky surrounding the island with gold paint. "What are we missing here?"
"You never know."
"Hey! I'm not going up there after you clowns!" They looked to the foot of the hill and saw Ranoal waving for their attention. "Chief Lanneoru wants us! The order came down! We gotta reduce the oil usage!"
"Geh. Stupid oil." Doanu began to tromp down the slope. "What a pain. Where do you think those 'whale' things come from, anyway?" "I don't know. Maybe we can find them someday. With waterships!"
Doanu snorted.
Finis