[Captain Drastic]

I'm not recommending all players have the same style of play. Some just don't have the time to go into all the details.

From "The Submarine Commander's Handbook"-
29.) The danger of a surprise attack exists, in particular, when the submarine comes to the
surface, especially after traveling long distances at considerable depth. When coming up
from a considerable depth, an all-round sound location should therefore be carried out at a
safe depth, where the submarine cannot be rammed; i.e., at a depth of approximately 20 m, at
"sound-location speed." Next, the submarine should go rapidly through the danger zone at
periscope depth, with the periscope raised; careful all-round look-out with and without
magnification - submersion up to 9 m, depending on the weather, then lower the periscope
altogether (see No. 23) and surface at high speed. The manhole of the conning tower is
opened as quickly as possible, and the commander - with, at the most, one man who is
especially good as a look-out - goes up. It is not until the surface of the sea has again been
examined with binoculars, in every direction, that the compressed air cells can be completely
emptied of water.

This seems to imply a "safe depth, where the submarine cannot be rammed; i.e., at a depth of approximately 20 m" , so 20m or below is safe cruising, commander's discretion.