Grumpi, one small issue with your ploy to use a high altitude search platform: clouds.
Well, it is not cloudy every day, and the search time is
much longer per mission, so you will gain a lot even if just flying every other day. Plus, low level clouds, and clouds that it rains out of are a problem for regular search aircraft as well...
One day with a 12 hour "shift" in the search area is as much as 6 days with just two hours each for a regular search craft.
Plus the swath width can probably be larger. At these very low altitudes that regular SAR craft fly I think you cover not much more than half a mile either direction (looking for stuff the size of a flock of seat cushions, containers etc.), One mile at most. Consider also that a lot of objects like suitcases, composite components etc. (and bodies), will be mostly submerged, and therefore will be much, much easier to spot when looking straight down rather than at a shallow angle from a low flying craft.
So if the camera on a Global Hawk can take images fast enough and the harddisk is large enough, I think you could cover up to four miles swath width at a much more useful visual angle and very high resolution. From, say 10000 ft AGL. That would give up to 10 or even 20 times the area covered compared to a low flying search plane, per day! Plus the risk of an observer missing an object zooming by is much reduced if there is enough computer force / image analysts available.
(if lower res is enough, swath width can even be increased much more)
If there were a chance of metallic objects floating, the thing could even stay for the night time and during clouds, and radar search for >20 hours per mission, with possibly even larger swath width. But I am not sure if the RQ-4 radar would pick up an object such as a mostly submerged LD3 container. On top of the water definitely, but submerged, not sure.