While you do not mention which port of call stop you are speaking of, in most instances the ship visits a port for 8 hours or so; this should be enough time for you, but I would allow plenty of time to return to the ship as that is on you to be back on time prior to departure.
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If you are asking about making your own reservations for shore excursions, you can usually find local tour operators at the shore who will offer you a variety of excursions. It can turn out to be cheaper but on the flip side, you are responsible for the excursion and if you are not back on time, the ship will not wait for you and you will have to fly back at your own expense.
I would suggest checking online and finding good tour operator before you sail. You can make the arrangements before your sail and that would save you some time.
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