So Mayhem is actually not human.
She is a selkie! If you dont know what a selkie is, heres some info.

”The legend of the selkie is Ireland's concept of a mermaid. The selkie – also called the seal people, the sea people or the mermaid – is a marine legend that tells of people who are half fish, and half human. In the water, they are seals, but on land, they shed their skin and take on human form.”

So Mayhem is a selkie that came to land long ago. She was seperated by her family from the tides and was swept to shore. The family tried finding her but they never could, so now Mayhem’s Family thinks she is dead.

Selkies have a fur coat that allows them to go into water and return to their seal form. Mayhem does have one, but she only uses it for drastic measures, like if someone needs help escaping a threat.

She can swim very fast in water, like a seal, and also has paws like a seal when she turns into one.
But if a selkie dawns their fur coat, they cannot return to the surface for 7 years. So Mayhem tries not to use it.

Her powers vary from time to time, but she is always easily able to use water manipulation and run/swim at rapid speeds. She can also manipulate her body to a selkies without using her coat, but it will damage her stamina and agility greatly. In my words, its like if Tanjiro Kamado used the Hinokami Kagura too much.

This is her canon seal form.

And this is her normal design as a human!