+Min Detonation Time or something similar, such as...I dunno, may be +Arming Distance(?) would be perfect for a submarine battle mod. Torpedoes have safeties so that they don't go active until they're a certain distance from the launching submarine, so that they won't lock on to the boat that launched them, and don't arm unless they've traveled a certain distance as computed and set by the fire control computer on the boat.
A missile that simply breaks up on impact if it hits before its warhead is armed would be great for say...a Hunt for Red October mod mission if anyone wanted to make it, or a generic submarine combat mod. (Can't you just imagine the voiceovers for one of those?) Though it would be a totally different style of combat. No primaries. Only secondaries in the form of torpedoes.