Nice concept. I kinda liked controling the submarine (^w^) I only explored 30% of the cave in 20 minutes, but the game seems to let me continue after that (~^^)~
I however did cut/lower the sound after 4-5 minutes because the sonar was kinda repetitive. You probably might want to add some objectives/mechanismes... or having very gorgeous level design and art direction to keep the exploration interesting. But ! As you said, this is just a concept game (~^^)~
Anyways, thank you for making this concept game (^w^)