You find the first doubloon under the covered bridge to the right of Main Street, by crossing under the bridge right to left.
You begin collecting many more when you are trading among the islands with your ship. Some can be found floating in the sea as "salvage" (mainly around and above Dragon Cove, and on the reefs between Parrot Port and Golden Harbor). You can earn them by buying and selling the four cargos at the five trading ports. You can earn lots of salvage once you have a ship with cannons, by fighting and sinking sea monsters and pirates.
(see related question)
You can reach the doubloon from the right side of the bridge. Cross over, then go down to the river and cross to the left. The doubloon will be in the sand there.
The doubloon found under the bridge is used to buy the chicken feed in the general store.
Cross the bridge, then go down to the water. You can cross the river to the left, and you will find a doubloon there.
Cross the bridge, then go down to the water. You can cross the river to the left, and you will find a doubloon there.
The girl who made a wish with a doubloon is on the right side of the bridge. The doubloon is in the water under the right side. The woman whose husband is missing at sea is on the beach to the right of the governor's house (far right of the island).
You can find your first doubloon under the bridge to the right of main street. Go across the bridge, down, and left. Once you have a ship (the raft is first) you will collect doubloons by salvage and trading.
Just right of main street at Fort Ridley. Go left under the far side to find a doubloon.
You find your first doubloon under the covered bridge. Go down the right side and cross to the left. Other doubloons are acquired in the sailing stage of the game. (see related question)
you cross the bridge on the right (after you go in the island) go down, and go left. get the doubloon. Sorry i can't describe it very much just go to Youtube put in Skullduggery Island cheats and you find it.
Apparently at the covered bridge over the river (or estuary). If you look under the right side of the bridge, you will find the doubloon the girl used.
You go east and you go on the hill go left. Go down the rocks go down then go left Then there's the doubloon.
Fort Ridley is the first place you go on Skullduggery Island. Its impossible to miss it.