[UKönig]

Ask your Navigation officer to sound the depths with the echo-location gear.
Keep sounding at regular intervals and jot down the number he tells you. In that way, you can keep track of the shallow depths.
As far as I know, there is no way to increase diving angle manually. You can press "P", or "C", usually configured to execute dive commands which take you down to a specific depth and level off. Pressing "D" will also dive the boat, but will continue going deeper and deeper, until you press "A" to "maintain this depth". If you do not, you can accidently take your boat too deep, and invite pressure hull failure.