You can only get a few at first, because your cargo area is limited. But once you have 3000 doubloons, you can buy an actual ship that collects more salvage. You can buy bigger ships as you can afford them, and even take out loans at the bank at Golden Harbor (these need to be repaid before a larger loan can be made).
To gather a lot of salvage once you have a ship with cannons, go to the area above Dragon Cove and sink Sea Monsters.
It takes approximately 25-100 "days." Everytime you land is counted as a "day" in Skullduggery Island.
You can get doubloons by sinking other ships, monsters and by selling cargo.
keep trading
the cannoneer is 20,000 doubloons.
it costs 1,000,000 ( one million ) doubloons. that's a lot of money! trade a lot, and EVERY day and it wont be long before you acquire it.
you have to get doubloons by trading grain silk medicine and spice (go around all of the islands on skullduggery)
Earn 500,000 doubloons then get a loan at Golden Harbor at the bank .Or you can as many as Grains as you can then sell it at Pirate Outpost until you get 1,000,000 doubloons.
it costs 20,000 doubloons
All you do there is to hire the Cargo Master, when you can afford him (salary 15,000 doubloons).
"Cash" on Skullduggery Island is your doubloon count. You earn doubloons by salvage or by trading, and you can borrow some from the Golden Harbor bank. When you buy cargo, you spend your cash, and when you sell it, you make a profit. If your ship is sunk, you lose aprt of your doubloons and part of your cargo. You need doubloons to buy larger ships and hire skilled crewmen. (see related question)
When you have enough doubloons, from salvage or trade, you can buy a larger ship at the Shipbuilder on Dragon Cove.
Usually Parrot Port, for between 32 and 40 doubloons.