Mines are most useful at strategic, psychological weapons. For example, if you were to place mines randomly in an enemy harbor, and then tell CNN that one of your enemy's harbors was mined--but don't bother to say which--you'll pretty much shut down all seagoing commerce for a few days.
Their tactical applications are somewhat limited. The easiest way to get mines to make an impact in a tactical situation would be to use the minefield to slow the advance of enemy forces through a specific area. The minefield--assuming the enemy finds out about it--forces the enemy to slow down and use HF sonar when passing through or to rely on MCM ships. This in turn increases their vulnerability to your other forces and gives your side more time to respond. This sort of minefield should cover all routes of approach that you intend to deny and should be long enough that the threat persists for as long as possible.
If you're actually trying to get kills in a tactical situation, it is difficult to make mines effective. You will need to figure out where the enemy will be passing through and have enough mines to be able to form a barrier across that path. It is crucial that the mines are placed very close together so that the enemy cannot go between them. For this kind of minefield, the more concentrated, the better. It should only be as wide as it has to be to block the path, and should only be a few rows long. Pray your enemy doesn't use his HFS.
As far as depth goes, that depends on your target. Mines for skimmers should be shallow. Mines for subs should preferably be in shallow water where you don't have to worry about "missing" in the 3rd dimension. Outside of shallow water, you'll have to build the minefield in 3 dimensions instead of 2.
LW/Ami will soon feature the CAPTOR mine, which will be quite effective against subs in tactical situations. The effective range will be mainly a product of how loud the target sub is.
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