It is definitely in the fishermans favour to collect anything they find, I read in the last couple of days about a previous crash where a fisherman found what he thought was a piece of plywood that turned out to be part of a tail from a similar crash in the 90s.
Boeing rewarded the fisherman by paying him $5250 for handing it in.
Having spent a good portion of my life in countries where people live a hand to mouth existence, I have witnessed many times the looting which occurs after a tragedy. It's sad that some people live in conditions where an airplane crash represents the opportunity of a lifetime, but it's true for a large part of the world's population.
From what I've read here, the area is filled with small fishing boats, and my opinion is that no one would report a crash before scavenging whatever debris was left on the surface.