Hey - great effort on following the reference game. You really seem to have the mechanics of that format nailed down. I love the setting and theme, I think it works really well.
However, I had issues with the slow speed of the sub - might be my laptop but it was painfully slow to turn and move, and that made it a bit of a grind just to get some bubbles. I think maybe you should introduce more things early to speed up the sub process and perhaps give some advice and tooltips - like where do we go, what do we do to get upgrades and how and where do we shoot etc.
I really liked the art style, I think you can do much more with it. You could have the sub captain's profile pic up, perhaps saying things to help clarify what is going on. Then maybe you could have other crew members pop up in similar pics updating us on what is going on, what is near, how much further we need to go before next stage, updates on weapons etc etc and also maybe ground control coming in with tips and info etc
Also, I feel like the atmospherics and art are doing a great job of conveying the situation, but the floor makes it look like the sub is grounded a lot of the time. Maybe add some objects on the seabed for perspective, or maybe a shimmering water-style effect over the top.
I do love where you've taken the theme, though, it really suits it and you are defo on the way to capturing the feel really well, just needs a bit of juicing and polishing. I'll definitely give this another go and see how far I can get, I did enjoy playing, just the pace was way too slow I thought.
Seriously, though, fantastic effort in such a short time ;)
*I think my laptop is running a bit slow actually so will reboot and play again ;)