An environment filled with active fishing boats in relatively shallow (<100m)
water sounds to me like a nightmare environment for any type of sound detection.
Just because the noise clutter from these ship is principally at lower frequencies does not mean the higher frequencies are clear. Rather there is a comprehensive fog of noise that blurs everything.
Imho, they cannot locate anything using the pingers on the ELTs unless they are really close, within a few miles at most, so finding debris visually is an essential prerequisite to knowing where to look. The range could be increased if the authorities can enforce a fishing holiday, but that would be unprecedented
afaik.