Okay, so in Marinetale, instead of monsters being sent to the underground, they were sent deep in the ocean. How you got here- you were a child on a ship an it sunk, and you were the only survivor.
It's pretty much normal but underwater. You can still breathe normally and move normally, etc. you also cannot get wet. Nothing will float up in the water unless you force it too, and you can still eat food normally, etc, because it won't get wet either.
The outfits of the characters are a bit different in color, but they mostly have the same personalities.
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Snowdin: Some people call it 'Icedin' because it is under a huge iceberg. The snow on the ground is ice underwater, but you can still walk on it normally.

Hotland: Full of underwater volcanos, so it makes the water hot.

Waterfall: This is an underwater cave with air pockets.

Ruins: Lots of walls and puzzles but no roof. Some humans think that this place is Atlantis.

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I might draw the characters soon, because they have different outfits.
OuO