[Iron Budokan]

If a DD or plane sees you it's not problem to ping the bottom once as you start your dive. They know you're there anyway so it's not like you're giving away your position.

The depth marker given wtih GWX to compare to the blue on your map is fairly accurate, too. And make sure you ping the bottom when you're in a colored area of the map while you're still some distance from the convoy to give you some idea what the depth beneath your keel is like....