We used to get a green card (I don't know if that's what it's called, but the card was green) years ago for our big bag of camera equipment. Since we were carrying 1-2 SLR bodies plus 3-4 lenses and accessories we thought it best to have it, though we were never asked for it. We had to go to some office, show the equipment and the purchase evidence, and they gave us this green card. Every time we bought new equipment, we would go down, and they'd add it to the card. While it's always possible to tell customs that you brought that equipment from home, you have no way to prove it, so they don't have to take your word for it. And since we did occasionally buy equipment when travelling, which we declared, we wanted to make sure we didn't get assessed duty at a later date.
There was a case recently where someone went from Canada to the US to buy a $115K watch and didn't declare it. He was stupid enough to send the empty box back to himself, and it got flagged at customs, so he was assessed both duty and penalties of around $35K. The problem with trying to just wear it back and claim you brought it with you is: (a) maybe they don't believe you, and you can't prove it; and (b) if you ever want warranty work on that, you need to have evidence so you need the receipt and if they find that, you will be charged penalties as well as duty.
We've looked at the watches on board, and, quite apart from duty, we didn't find the prices to be all that great compared to what we would pay at home. And the selection is limited. If we really want a high-end watch, we can order one from our local retailer who carries them, who records the purchase and who can provide or facilitate any warranty or repair work on them. We've not been impressed with either jewellery or watches being sold on board ships or in the ports, both in terms of selection and price. And on our last cruise when we went into one of the stores on board just to be nosy, we got pounced on immediately and had difficulty shaking them off.