Because I have 30 days to reach my 100 words in 100 days, I thought that I would use these last 30 to actually write something. It’s not a big deal. I can write 100 words on the same subject over the next 30 days.
I will use the next five days to plan, including plot and characters, and then the 25 days after that to write a story that has a beginning, middle, and end.
Here we go…
What would be cool? Breathing water would be cool. An underwater city would be cool. Sharks are cool. Shipwrecks and treasure are cool. Mer-people would be cool. Manta Rays and glowing fish are cool. Cenotes are cool. Caves are cool.

Theme – this is a hard one. What do I want the story to be about? The ocean is our mother. The ocean should be respected. Practical Creative Writing.com has a list of themes. Lets go with Survival (man versus nature, nature versus man), Greed/Ambition, and Discovery (new places, inner meaning, treasure).

Darcy Pattison.com has some plot templates.
Maybe a combination of

Quest. Character oriented story, the protagonist searches for something and winds up changing him/herself; Adventure. Plot oriented, this features a goal-oriented series of events;

and

Discovery. You know those secrets you’ve buried deep in your past? This story digs around, exposes secrets and watches them affect the characters.

Possibilities:
A marine biologist goes out with a shark hunting group to gather data for her ph.d. The men she goes out with are thrill seekers instead of the scientists they claim to be. There is also a treasure hunting group in the same area, looking for a sunken ship that has lots of gold. There is a mer-shark (man who can turn into a shark? Shark that can turn in to a man?) who is trying to protect his people from discovery. Evidently the treasure hunters and shark hunters are getting close to finding the underwater city. Not Atlantis but a sister city.

Setting: Ocean and coastal towns. Maybe Florida and the Caribbean, or South Africa, or Australia. Or all three.

Magic: The mer-shark can change into a man and exist out of water for a set period of time. He can swim into the deep waters, breathe both fresh and salt water, he has bioluminescence. In shark form he has a powerful tail swish that can knock things over. Ane then there’s his teeth, his ability to use the Earth’s magnetism to find his direction, and his ability to sense electrical fields.

Maybe there’s an evil scientist who wants to experiment on the mer-shark?

Goals:
Mer-shark must protect his secret (his civilization)
Marine Biologist must study sharks to get her Ph.D.
Shark Hunter Scientist(s) must get glory by finding a rare shark species.
Treasure Hunter(s) must get the treasure, which happens to be in the same area as the rare sharks.