Underwatertale is just like the original Undertale but Mount Ebbot is replaced with The Great Ocean, Frisk becomes a mermaid when she enters The Great Ocean, and a Seahorse named Shelly joins Frisk in her adventure. (Also, Asriel is a girl and CAN be saved. Because in the original Undertale, you can't save Asriel.)
Here is the Story:
Long ago, two races ruled over Earth: HUMANS and MONSTERS.
One day, war broke out between the two races.
After a long battle, the humans were victorious.
They banished the monsters deep underwater, and sealed it with a magic spell.
Many years later...
THE GREAT OCEAN
Legends say that those who enter The Great Ocean never return.
Here are the Characters:
Frisk: she is a human in the surface but transforms into a Mermaid when she enters the Great Ocean.
Shelly: She is a Seahorse who helps Frisk while exploring The Castle of Rivers, and helps Frisk defeat The River Guardian. Now she helps Frisk on her adventure. But there are two sides to her. In the neutral run, When Frisk defeated The King of Seas, Shelly kills Him and his soul as well and absorbs the human souls. Her neutral run final boss name is Shelly, The Monster of the Ocean. In the pacifist run, When Frisk's Friends came, Shelly intervenes and talks the dialogue Flowey says in the Undertale pacifist run. And becomes Ashley Dreemur. (Female Asriel) in the genocide run, Shelly is just like the genocide Flowey.
Aqua/River Guardian: She is Toriel. She is the Protecter of The Castle of Rivers. When you meet her the first time, She is the River Guardian. But when you meet her the second time in the pacifist run, She becomes Aqua.
Sasha Seaweed: She is Napstablook in this AU.
Sans: He is the same but a deep sea diver.
Papyrus: The same.
Splash: Sans' Pet Dolphin. Papyrus tells Frisk that he found Splash while exploring "A Human Boat."
Coral: She is Undyne in this AU. But is a mermaid like Frisk.
Danny: He is Alphys in this AU. But he is an Octopus.
Seastar: She is Mettaton In this AU.
George/The King Of Seas: He is Asgore. In the neutral run, he is The King Of Seas. But in the pacifist run, he becomes George.
The rest is a WIP.