BB is right, simply add it to your contents insurance. It may, however, already be covered under your current policy and it would be worth checking your policy out first. For example, my policy covers me for jewellery - any single item - valued at less than £1,000. For any jewellery that I have valued at more than £1,000, I have to inform my insurer and they add it onto my jewellery schedule for an additional premium.
Depending on the value of the watch it shouldn't cost too much at all to 'tag' (pardon the pun) it onto your policy - that's if it's not covered already. Needless to say, ensure that you keep the receipt for the watch in a safe place, as if you ever did need to claim, you would need to produce this.