Alright, gave it a good try, beat 5 or so missions. Took some notes:
You really drop the player in without much warning - I started and immediately got mobbed by nautiloids. Maybe don't spawn enemies too close to the player's spawn.
Screen feels very small for player's speed and how much momentum you have.
Shooting up and down feels kinda jank, because unlike horizontal shots you can't do it without moving - try having the projectile go in the direction of the player's last input instead of defaulting to the forwards facing?
While frantically shooting while running away and turning, I managed to get a torpedo to sit basically still in the water. It's cool that their starting velocity is based on the player, but they should probably accelerate to their max speed relative to the environment.
The pickup sound for the treasure chests is incredibly shrill and annoying.
The transition to the mission complete screen is super sudden and jarring. Maybe have a countdown on completing the objective?
Cost of upgrading doesn't seem to increase until after next mission? could be a bit broken if you save up a lot.
Combination of the booster gadget and the drill is lots of fun, but doesn't seem particularly strong compared with just spamming torpedoes.
Uh, overall impression, it seems nice and polished but a little basic. Add more enemies and stuff and you're good to go imo.