I've booked private sightseeing tours for most shore excursions, and I was wondering if there's a specific time we have to be back on board the ship? I guess 1 hour in advance should be good enough, right?
The "all aboard" time will be noted in each day's Navigator. Getting back on the ship one hour in advance of "all aboard" time will be fine, but you need to be sure allow for traffic and anything else that may delay your return, because the ship can leave right at the designated time. Experienced tour operators should be able to get you back in plenty of time, but allowing at least an hour is wise.