You take a form that can enter the water

You resign yourself to what just happened. You want explanations, or even to seek revenge against those damned tentacles.

You think about becoming a form that can breathe underwater and move through it.
A form materializes in your mind, and you focus on reproducing it in your body: You transform into a mermaid, a type of swift, graceful, and beautiful aquatic humanoid.

Instead of legs, you have a large fish tail. What you do next is dive and start swimming toward the depths, searching for those damned tentacles.

I’ll keep working on this, it’s still a WIP (work in progress). I’ll be reading any feedback you have, so feel free to share your thoughts!