I doubt you would get in trouble wearing a fake watch, as tons of people do, and you can always say you thought it was real, honest mistake, it was a gift, you got ripped off, etc. etc. It isn't illegal to purchase rep goods in most places in the world.
Getting caught with two or three is a lot more iffy.
I would do some research on the laws in Thailand about that kind of thing before you go for it. Most likely they just confiscate your reps and send you on your way, but I would check that out first.