Here's what I do in shallow waters: I put on my SCUBA gear and exit the boat by pressing F12 to perform a visual inspection of depth under keel.
OK, some may call it cheating, but I say if Kaleun von Stumpf's hobby is SCUBA diving, then what's the problem?

Also, U-boots must've had better / more detailed depth charts than the one in the game... right?
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Bernaaaaaard!
Current games: UBOAT ... Victory at Sea Pacific ...
Ye olde games: SH3 + GWX3 Gold ... SH4 plain vanilla ... Steam and Iron ... Jutland ... War Plan Pacific ...
...and many board games!