@RogB personally I'd never buy a boat without a test run. Engines perform differently when under load than when just sat in a bucket idling. Any honest seller would understand that and make the effort to show off their boat by showing you what it can do. If it's newish I'd expect 2 sets of keys and the kill cord. Also a first, second service receipt or stamp in the service book.
If the seller refuses to take you for a test ride, how do you know it's their boat and not lifted off a driveway somewhere?? Check all ownership papers and make sure the name on the paperwork matches the sellers ID!!