When you beat the level "Dead Men Tale No Tales" you will unlock the boat for use whenever you go back there. Now I love to shoot the ships and sink or capture them. So you need to board the ship from the dock when you are Sly. Now go to the wheel and press circle and Sly will poliot the ship. Now just go out of the bay and find a boat to attack. I have averaged roughly 200 coins for a sink and somewhere around 300-500 coins, it all depends sometimes. If you get hit, just go to the hole in the deck and hit it with your cane and when you finish the attack find a health chest in the water. Happy Hunting!