I have purchased two Citizen watches on a ship and I'm very happy. The discount is no longer 40%, but 35%.
That said, I've checked pricing, and the ship pricing on Citizen is the best I've found. Also there are so many Citizen's models, many use the cheapest model price as an example of bad ship pricing. They are not all the same!
At the end of the day, I save at least the 6% state sales tax.
The vendor that runs the shops does report the purchase to customs so don't try to hide it.
Buy your watch EARLY in the cruise to make sure it works! They will size it for you.