In order to unlock the three secrets in this game you need to meet ALL of the following requirements:
-Over 2500 kills
-All crew members found
-Over 12 non-storyline ship battles, including the 4 extra ''monster battles''
-All bounties defeated and reported
-All discoveries found and reported
-90% or higher treasure discovery
After this you can find the Gold Hamachou above the tunnel which connects mid-ocean to the sailor's island area, you can buy the Sky Fang from the mystery merchant, and you can go fight Air Pirate Vigoro at Crescent Island.
(Note: many of these stats can be checked at Hamachou Island.)