USAF1956
1. If the US or another country had a SOSUS (or similar) system near there, wouldn't it be possible to listen for an anomaly that could be the plane impacting the water.
2. There is an extensive SOSUS sytem in that area that would have detected a noise of that magnitude,
FWIW,
SOSUS was/is mostly a deep water system, not a good fit for the comparatively shallow water of the South China Sea. Beyond that, were there some hydrophone array in the area, a surface anomaly (short duration acoustic event) may or may not reach a given hydrophone due to the varied paths sound takes and the high ambient noise in that region. My estimate (very out of date, been some years since I did real ASW) is that there is no such hydrophone array in that region.
Hoping that SOSUS could give a datum for this search seems low to zero probability.