Cruise ships offer many options for shore excursions during their cruises and they can often be a highlight of your cruise. However I have found that the prices when booking on the ship are often excessive. At many ports of call local entrepreneurs line up on the dock to provide alternative tours.
One consideration you need to be aware of is that if something goes wrong with a locally organised tour and you are late back the ship will not wait for you and it could lead, as it did on a recent cruise I was on, to an expensive 'catch up' trip. The ship has to wait if a ship organised shore excursion is delayed.
In many ports it is often not really necessary to join a ship organised shore excursion. In cities I have been to on recent cruises such as Melbourne in Australia, Wellington in New Zealand and Bar Harbor in Maine, USA walking or a local bus will quickly take you to the main sights after disembarkation.
I have found that some shore excursions really try to do too much. For example, on a visit to Melbourne a tour was available to the famous Twelve Apostles south west of the city but this involved a four hour drive each way with only a short stop at the attraction; many elderly passengers claimed they were exhausted by the journey.