cactus

Hello forum i'm new and sorry about my english because im italian. My question is about a film i see yesterday. one submarine hit one rock and start to fall deep. 3 pepole of the crew use an escape blankit and reach the surface. my question is you cant compress the wather but you can compress air. If you compress air become heavier and i think you cant exit from the submarine and reach the surface whitout a motor. There is a formula that I can do to find the depth where the air becomes heavier than water?. im sure about this because i use to dive and when the tanks are full they sink when are empty they floath. and my friend archimede thell me this is true .. ty !!