Try going into the control room view while on the surface and look at the big depth gauges (the fine scale ones that go to 165 feet) and you'll see the depth varies around 15-17 feet, sometimes to 20-22 if the weather is rough. Just took a look at a pic of a real one, minimum number is 15 feet. Check the game; yep, same.

The sub captains had crap for charts, often using 20 year old National Geographic map inserts to figure out where they were. Soundings? Forget it! What we have in the game is magnitudes better than what they were stuck with.