Consider an alternate version of the analogy:Why assume the beach isn't designed? You already assumed the beach isn't designed, and, the watch you found was obviously something made somewhere that you would assume its just lost, or something.
You put various things into the premise, that are pre-concluded.
You're walking along a beach when you come across a watch. You move the watch aside, scoop up a handful of sand, and recognize that the sand was designed.
Does that analogy work?
The watchmaker analogy assumes that there are "hallmarks" of design that a watch has but the things around it doesn't. If you are telling us that there everything is designed, then this means we have no hallmarks to tell a designed thing from an undesigned thing, which makes the analogy fail.