I absolutely love the visual design of this! The minimalist pixel art is very striking, and the monochromatic color palette both adds to the mood and allows the yellow power ups to really POP, making it very clear that they were a goal to move towards. I thought the level design of the first section was very well done, basically forcing the player through a loop to get the thruster upgrade before opening the world up significantly with the ability to go in the sub.
Unfortunately, there were definitely a few elements that made the game more frustrating than it should be. As others have mentioned, the slow movement, while definitely on theme, makes it feel a bit like a chore to play. I died once on my way to getting the thruster and was very annoyed that I had to do that whole loop again, since it was a bit tedious and not particularly exciting. But once I got the sub, movement felt a lot better, although since the world opened up so much, it did make it a little daunting to try to choose where to go next. Also, while the color design is excellent, it did make it occasionally difficult to determine if some stray pixels were spikes that would damage me, or just part of the environmental aesthetic. Maybe you could choose a separate color for things that would damage the player?
But overall, this is a really impressive piece of work!