Interesting idea, would've been nice if you had more purpose than just "dive as deep as possible", maybe collecting treasures or something. Maybe even being able to upgrade your submarine over time (although that wouldn't have been that easy to implement in the short time we had but just an idea), to be honest i don't really know, but something was missing.
I think what most people had a problem with was how slow the submarine was moving.
Really liked the art though! Keep up the good work!