Would this be a good time - if they haven't done it already - to lower a duplicate ULB (preferably with a dying battery) to the sea bed where they got the original readings?
Then, a) see if they can detect it, b) if they can, compare the readings and waterfall graph with the originals and c) check any signal deflection which they can then factor in to a new search area.
It would give them confidence (or not) to pursue the search in the current area.