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You want to search the golden monkey. Walk to the lower left corner of the ship to leave it and explore the island. Talk to the Fatso standing there and learn about the cave and the gold. There’s a gray spot that leads to a cave if you use it. Pick up the KNIFE that’s behind the crate on the floor.
Walk to the left out of the cave and use the ship to enter it again. Walk to the lower right corner of the screen to enter your quarters, fight with an octopus and get a large piece of MEAT from the fridge. Leave the ship on the left lower corner again and on the island give the meat to Fatso. In return you get his GUN.
A good thing because you lost yours in the fight with the octopus. Go into the cave again and use the gun on the black wall on the right. The bullet will ricochet and hit the crate, causing it to crack open. There’s your gold!
Game source: A copy of the game was found here  on the internet.