The setting is fantastic! I really like the idea here, and the ambiance you managed to create with just greybox assets is great! Really felt like wanting to play more.
On the gameplay side, I peronally felt that the walk speed was too slow. It is great for areas where we need precision, though, but those seems to be not the majority of the game. So what you could do here is sawp running with walking. Make running the default speed, and hold SHIFT for walking, for when we need that extra precision.
The jump felt quite floaty, which is fine, since the game setting is underwater, but then, please give me more air control. I feel Isabela should be able to "swim" a bit with each jump, and reposition herself.
The first harpoon section, I couldnt read the tutorial fast enough to understand that LMB shrinks the thether. So please make the instruction linger a bit more, or have a menu where I can check the info again.
Speaking of cutscenes, we can control the character during them, and that can cause wierd interactions. You should take away player control while a cutscene is playing I think, for safety.
I played up to checkpoint 21, but there is a wrong placement of the checkpoint, which respawns me on a freefall to my death. So you should probably get that fixed.
Lastly, please add controller support. This seems like the right type of game for it.
Keep up the good work!!