He's probably plotting the aircraft on the nav map. Most people use it, and TMO's designed to some degree to have it on. In case you ordinarly check "no nav map updates" as quick as i might click, "realistic fuel" the nav map was umm.. nerfed. It no longer displays nationality, ship type, velocity vectors, sensor information, etc. All you get, is a little black dot to mark the contacts position. Everything else you have to do yourself.
Now as cruising goes, staying down at 400 feet is overkill.
Personally, if i cruise submerged, i'll do it at 165, sometimes to 200 feet just to clear the first dial if im expecting trouble. But normally at 165 feet. I've never been hit at that depth. They carry two types of bombs, one explodes on or just near the surface, the other, an air dropped depth charge, will detonate at around 147 feet, give or take 3 feet. At 165 feet, it might blister the paint if it goes off at 150 feet, but i doubt it would do anything major. I could be wrong though, some have said they got it pretty good at that depth, but i never have.