I don't think everything needs to be used all the time to be a worthwhile part of the game; but the sub ping is very rare and probably hardly of use even if you have it available.
My own suggestions for an improvement:
Have it given automatically to one of the allies factions (France say) immediately and permanently as soon as they build a shipyard, on a countdown timer where they can use it every 5 minutes or whatever. Not all maps have water but if you are building a shipyard that should discriminate a map where it may be useful to have it. By "permanently" I mean they retain the countdown regardless of whether they keep their shipyard or it gets destroyed or sold.
During the sub ping state, allow helis to attack subs still submerged, *but lower the damage* done to maybe 40-50% of the damage when they are attacked on surfacing.
You could still get them by spy infiltration for any faction.
