You need a large f/w UAV capable of covering about 15,000 square miles 50-80 nms ahead of the line of advance. And capable of launching weapons. This roughly replicates what organic fixed wing could do. The P8's would be much further out 100-200 roughly. This constitutes layered defence and the desired detection, location and prosecution of the target would always occur way beyond the screen.
MQ-8B Fire Scout
The Fire Scout (even the small one MQ-8B) could deliver sonar bouys. With a 600 pound lift, its not unbelievable that it could also be armed with something like a Mk-54 if required. (not really needed IMO) this is really best handled by MH-60R/P8's.
Even something like the proposed OPV could operate 2 or maybe 3 Fire Scouts each (with 8hr endurance thats 24 hr coverage, just, for short periods). Thus enabling a nearby frigate/destroyer to carry a ready to go, armed MH-60R.