To be fair Dai, your sonobuoys in the central Atlantic were designed to hear a submarine a couple of hundred meters below, within a km or so, and not a pinger from 4km down.
Emeraude also has a much larger sound collecting area than those buoys.
You're absolutely right that listening for data recorder sounds in 100-foot deep water full of shrimps, raindrops and shipping, large and small, is not going to be easy.
In water as shallow as the South China Sea, towed arrays are also going to come into problems, as they are going to be in water whose depth is comparable to their length.
I would bet that a trawler has to be the most likely to find the wreckage.