In training before the war began, U-Boats operated at a maximum depth of 50 meters even though their "safe" design depth was 90 meters. The fact is, they had no idea the boats could dive so deep as later became the norm.
So--when I start a career at the beginning of the war, I limit my depth to 100 meters until mid 40. This
simulates the knowledge the U-Boat commanders had. It also makes those 300 foot depth charge settings a bit more dangerous

.
Personally, I kind of get bored with early war when all I to do is get to 150 meters, set for silent running and then go do my grocery shopping.
This is not to say
I AM GREAT. It only proves that I have I have genetic material (resessive of course) that leans toward the masochistic

.
Try it and you'll find out why Germany also lost a lot of boats in the early war. And when you finally discover you can go deeper (I use 1 June, 1940) you can congratulate yourself on having survived the
Questionable Times.
Happy Hunting!!!