"Surely there must be submarines there - anyone have an educated answer/guess ? "
There very well could be, but experience from the search for AF447 seems to say that submarines get in the way of towed pinger locaters, and vice versa, because of noise and cables. So even though it sounds impressive, that submarines, especially "Nuclear" submarines, are deployed, it seems that deploying purpose built equipment is preferable. Also, the subs would have to have sonar equipped to hear a 37.5khz ping from the ULB, which most submarines do not have. Of course, before any pinger locaters are deployed, I don't guess it would hurt to have submarines, or anything else, such as yachts, fishing boats, & etc, out there looking.