coagie, the nice thing about the deep ocean, well over a thousand fathoms, (my best guess at how deep this beacon would be based on the area being searched) is that much of the surface noise would be attenuated (bends back up toward the surface) and thus raises the potential Figure of Merit for passive detection. That said, getting the sensor deep enough to have a chance at picking up a comparitively high frequency beacon poses some problems.
mm43 has covered the rest.