Dubloon is a token which is sold by the bartender at the Beak Easy penguin club beneath the outpost. He won't offer dubloons for sale until after a player has defeated Dreadwing and restored the club. They're exchangeable using three diamonds to the bartender.
Once penguins return to the club they can be hired for one dubloon each to join the player's ship crew. Hiring these penguin mercenaries offers an alternative way to hire ship crew members without completing quests for NPCs.
Dubloons are worth more pixels (5000) than the three Diamonds they're exchanged for (3600), making them viable for enhancing the sale value of any spare Diamonds in your possession. However, Diamonds can be converted to 600 pixels in a Refinery, significantly higher than the 320 they sell for to a store.