They will die if you don't keep them hydrated. You can use the shower/tub or pool as well as going out in rain to hydrate. But you need to stay out of the ocean for 4 days. Then you'll turn human again.
You keep your mermaid from going into salty waters like the ocean. But to keep it hydrated so as to stay alive you go into the fresh waters like the streams, lagoons, ponds, tap water etc. Happy Swimming!