Well done! The game is short but polished.
Character is very cute. I like the idea of transforming into a mermaid and exploring and combat underwater. The music and sound effect is good too.
I think the momentum when jumping is a bit off. If the horizontal velocity during the jump can only increase when the movement key is pressed, it may feel better.
Overall, this is a solid entry and can expand if you want. Keep up the good work!