It is cannon island, just southwest of Mercay isle. but i will warn you it is like a puzzle that is fairly hard to get to eddo the first time.
Eddos Garage is on Cannon Island in the South Western Sea. Go to the sea chart and click on Islands until you find it then travel there. Once docked on the Island, Eddos garage is just to the right you can not miss it.
go 2 eddos place were you got da cannon and talk to him he makes you blow into the micraphone the harder the blow the less you have to pay for a salvage arm go 2 the sacred crest and salvage there
The salvage arm attaches automatically when you get it. refill its life at the ship yard on meracy island. It will cost you but its worth it. Click on menu on the bottom left of your touch screen and click salvage.