The game looks kind of relaxing, but the hit point limit can be a bit punishing with how quickly and wildly some of the fish move.
I had an issue with the upgrade shop, at first I was only able to buy the middle upgrade. Despite having enough money for the other two, the buttons didn't seem to work until after I had bought the depth upgrade.
The submarine should probably be flipped when moving left, currently it feels a little odd to move backwards. Some kind of option to return to the surface without piloting all the way back up each time would have been welcome as well.