I liked it, I was actually a bit sad it ended at the first boss :(
The game is visually very pleasing. I really liked the animation of the main character, you even added a crouch animation, fricking eye candy dude! The only thing I could point out in this regard is the motion blur on the sprite looks kinda bad, tho maybe this wasn't something intended and something from my pc.
The combat does the job. I like the possibility of having both a melee and a ranged approach. One improvement you could make is maybe making the melee attack hitbox a bit bigger, it felt a bit hard to hit enemies, even when they were kinda close range to the player :S
Platforming solid also, you implemented the feel-good juices of controlable jump height and zero acceleration on horizontal movement for more precision control, it is nice! I think the momentum during the jump felt a bit awkward, I believe that if I could control the horizontal velocity on air as if i was in the ground would make it feel tons better(like if I am not pressing left or right on, the character won't move forward).
The power-up you implemented is also very interesting, water movement feels snappy and fast, and that's the way i like it! The mermaid transformation sprite is also a very nice touch
The levels you came up with really nailed the metroidvania feel, at least I saw a lot of fricking paths along the way to the boss and was like "GAH I better remember to check these later" (and still am)
Overall, this was pretty solid! I believe if you ever want to expand this project, it has legs to walk! :)