A great game that ran flawlessly for me. Gameplay-wise it needs a few things.
1: Map, both large scale full-screen and a mini-map.
2: Upgrades for the sub such as cargo capacity, speed, battery efficiency, lighting, etc.
3: More levels.
4: A reason to collect the gems such as spending them on the upgrades, or once you reach a specific amount collected in orbit, you go to the next level.
5: Wave effects off of the sub when it's on the surface.
Add this list into the game with about 30 or so levels with additional DLC and I'd happily pay for the game.