Your ship will typically arrive in port early in the morning at which time you are free to go ashore. In most cases, you will be back on board the ship by 5:00pm to 7:00pm. This gives you more than enough time to enjoy one of the cruise lines' shore excursions, or, go off on your own to do some shopping, sightseeing, swimming, scuba diving, etc.
We highly recommend the cruise ship sponsored tours and excursions. In some ports, when you debark from the ship, local sightseeing companies (in some cases a local with a van) will be waiting to temp you with their own tour. Take our word for it, while these tours may be safe and in some cases quite good, it is always a safe bet to stick with your cruise ship's recommendation.