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.
It was the Adam Air Flight 574 crash:-
The fisherman received a reward of 50 million rupiah (equivalent to about $5,500) for his discovery
http://en.wikipedia.org/wiki/Adam_Air_Flight_574