With columns for seller, seller location, purchase date, retail price (if known), purchase price, model name, catalog reference, technical reference, serial number, warranty end date, six columns devoted to servicing dates (I'm in my 50's), and attached PDF scans of receipts, warranty cards, hang tags, a photo of the watch, appraisals, and other paperwork.
I use a spreadsheet instead of a database for anything that can be presented on a couple of pages, just for convenience. If I owned hundreds rather than dozens of watches, I might need something more sophisticated.
--Rick