Right. If you bump into an enemy in transit mode...
If you're going at high speed (SHIFT+ARROW), then you start at ahead full, at a deep depth. For the Seawolf that's 900ft at 30kts, and chances are whoever you're hunting will end up just a few thousand yards away, and they'll probably have the jump on you.
If you're transiting at slow speed (ARROW), then you'll be at ahead standard, medium depth (320 feet and 15kts for the Seawolf), with your enemies at a decent range. It's an okay shooting situation.
The best would be to sprint ahead of somebody and wait in ambush (NOT MOVING). It's hard to pull off, but when you do it you'll be at 5kts and 150ft for most subs, a very good position for an ambush. The detection will probably be at long range, and you'll be in a good position to intercept.
The exception is the ultra-quiet targets, Sierras, Typhoons, and SSKs. (Even the Juliet is modeled as quieter than, say, an Alfa or an Oscar). Especially SSKs; they have a nasty tendency to show up at extremely close range. It's nearly impossible to TMA them without an active sonar blast.
Okay, other hints...when you're shooting at others in an ice situation, you want to be careful that whatever you launch--'poons, Tomahawks, or Sea Lances--don't bump into something on the way up. Had it happen before.
Enemy torpedos generally snake-search upon activation; if they pick something up they go into circle-search. ASROC/SUBROC torpedos just start circle-searching upon hitting the water. Your Mk48s, however, plod in a straight line until they find something, at which point they circle-search if they miss. No snake-search, annoyingly enough. On the other hand, Mk-48s have a nice detection range, but their seeker cones are pretty narrow. And apparently they're quiet enough so that you could swim them above/below the layer as needed...they generally run at launch depth until they go active. Once they're active, though, they're totally out of your control.
If you shoot and miss, there's a good chance that the enemy will decide to go active to find you. Be warned.