There have been oceangoing robots designed to move with very little power by exploiting buoyancy and gliding on dive planes. Swarms of such small drones could be exceedingly hard to detect and could be made cheaply in large numbers.
The practical range of these things is literally ocean-spanning. Program these on roundabout courses and land them in random places on the coast. They would be very hard to track.
http://en.wikipedia.org/wiki/Underwater_glider
The practical range of these things is literally ocean-spanning. Program these on roundabout courses and land them in random places on the coast. They would be very hard to track.