Very Noble-Looking Watch probably has roleplay purposes. NPC John who sells this watch is a dishonest trader. He also sells a treasure map which we know is useless and probably a fake. Interestingly, we can blame him for selling fake goods while talking to him. Here is an example:
Player: treasure
John: Oh, now we're talking. If you want to find a treasure, you really need one of my highly accurate treasure maps.
Player: treasure map
John: Hey, hey, don't blame me for you not finding a treasure. If you can't read my map correctly, it's not my problem.
Player: watch
John: Hey, hey, don't blame me if you don't know how to handle a noble watch carefully.
These NPC statements may suggest to us that the map and the watch are useless in the game. It is also interesting to note that other NPCs on Treasure Island also suggest that the map purchased on the island is useless. Here's an example:
Player: Treasure map
Paolo: Get your own treasure map, this one is mine! I think the ones sold here are sort of... well, you probably need a real pirate treasure map here.