Spoiler (click to reveal)
There are the markers T1,T2,T3,T4 at the bottom. For 1-3 there is a passage that you move the sub through. Use the altimeter at the right to determine the depth at the place you're passing through. Then for T4 you escape to the surface which is at the top (altimeter at 0). Of course for each of these you'll need to be adding the number according to the altimeter directions.