"I wonder if it would have been possible to seed the ocean in the suspected crash zone with markers."
Well, you'd need to make sure the markers behaved like bits of plane, and the best way to do that, would be to use actual planes.
There's plenty of old passenger haulers around the world, rig them up with an autopilot and crash them into the sea in a grid formation, then sit back and track where the left-overs end up.
Simples!