Sea depth skybox is fantastic, so are the models for the Iso-chan (I'm coining this nickname, copyright, trademark, etc) and for dead crabs. Map is surprisingly vast, but relatively easy to navigate - though I would've preferred more landmarks for where I could and couldn't go. Jumping feels good, but I would prefer a bit more water control (air control but beneath the brine) - though it's a very minor nitpick, I didn't have any issue with current controls.
Harpoon throwing is good, appreciate the visual trajectory - though it flies EXTREMELY far with proper angling, and can be thrown almost across the map (which may not be optimal, especially with the grappling rope attachment). Speaking of grappling rope - it's a bit too finnicky, especially when you swing. I didn't have any problems, BUT I can see how it could become an issue. Also I would prefer some notification about when the rope is about to run out and I'm gonna fall - but that's probably in the QoL department.
Ambiance is nice, though there is some low freq ambient noise in the SFX channel. I would imagine it's intended since you're underwater, and God knows what's making that noise in the dark depth of the sea void - but it also messes up other sounds on the SFX channel (like footsteps).
It's a solid start for a platformer, and the setting is fairly unique. Looking forward to see it developed more!
Plz let me harpoon the fish to death.