[Sailor Steve]

In certain areas (around well-known and well-charted coastal areas) the actual depths were, well, known. The problem is that a submerged submarine has no way of knowing exactly where it is, and the only way to test the depth is to use the echelot, or depth-sounder. This was a form of active sonar, and anybody listening in the area could hear it. If the water was shoaling (getting shallower) they would have no way of knowing but by guessing. On the other hand, scraping the bottom wasn't usually problematic as long as there weren't too many large rocks.