you can only rename ships at skull island. at the docks, at the vendor guy with "ship renaming and dye" over his head. you can dye your ship and its equipment with him and you can also rename the ships. the amount of gold require to rename the ship or dye it depends mainly on what type of boat and what level it is (the more stronger or valuable the boat is, the more gold you have to pay to rename/dye it). for example i can have a royal navy ship, if the boat from port regal level(nautical 16/ level 25) it would cost me maybe 4000 gold to rename it. if its a boat from mooshu level(nautical 36-44/ level 42 and up) it would cost me 8500 or 10000 gold to rename it just because its for a stronger level.