Thanks for checking it out, glad you enjoyed!
The pressure gauge shows you how deep you are. By default, going below 300 ft damages your hull. In Level 3, you have to cross an area below crush depth to collect feathers, get through to the other side and rise again. In Level 4, you can pick up a pearl to upgrade your hull to dive deeper. In Level 5, you can pick up more pearls and allow your sub to go even deeper.