Mermaid Princess.

Astrid, Indah and Raya are three friends who get lost on the beach, and find a mysterious place while trying to find a way out. They passed out and when they realized they were in a cave with a pool of water. Astrid said that the pool was the only way for them to go home, but Indah did not agree because if they entered the pool they would turn into mermaids. Remembering that it was their only way out, Astrid and Raya convinced Indah to enter the pool, Indah agreed, the three of them then entered, not long after the light of the full moon shone on the pool.