Thanks for the response!
Just a message in a bottle in case anyone else has this problem - it turned out that the problem was, as mentioned in some posts here that I found by using 'search', is that, at least at the start of the war, your crew is very bad at detecting contacts. Thankfully, my PC is fairly old

, so I can use the cheat of running at 512X with the chronometer up, and when it jerks to a halt I can then use sonar myself. I hate to use cheats like this, but otherwise the game is basically unplayable

.
Thanks again Webster - also, I take it by the deafening silence that indeed you _can't_ edit the label on a mark, which is too bad.