What he means is the actual depth in the real world and the depth in the game might or might not be the same in any given area. For example the west entrance to Kobe harbor in Osaka Bay might be 42 feet deep in the real world, but 60 feet in the game. The fathometer in the game gives you the distance from the keel to the bottom as they made it in the game, and if they tried to make the exact real world depths everywhere in the game they would still be programming it and might have it finished 200 years from now.
I suspect they made the destroyers ignore the fathometer ping because of necessity to use the fathometer, since you look at a chart that says 70 feet you have to subtract 15 to 17 feet on the surface or 50 to 66 feet submerged depending on what sub type and what depth you're running at - after all, the actual depth minus the keel depth is the issue, zero depth under keel is where you actually hit the bottom.
The charted depths in the real world weren't accurate in WW2 for the simple reason that until submarines came along anything over 100 fathoms was "who cares?" The fathometer was invented sometime around 1925, prior to that they used lead lines cast at intervals to chart depths, and extrapolated between casts.
Simplest way in the game is to take soundings any time you're not in dark blue water, sometimes if the depth is too shallow for evasion I just let those targets go and look for targets in deeper water.
He who chickens and runs away, lives to chicken another day.
