There are many options for shore excursions to enhance your cruise experience. You can book your excursions through the cruise line provided options (our preferred method), you can book your own through a quality shore excursions provider that has a solid record (which we have partnered with the leader in this area), or you can wing it and grab an excursion with one of the local tour providers when you get into port (we highly discourage this option).
The reason we recommend that you book your excursions with the cruise lines (or tour providers) is because that these excursions and their suppliers have been carefully vetted by both the cruise line and the local government authorities to ensure that you are not dealing with a less than reputable or trustworthy guide or operator. The cruise line selected operators also are required to ensure that you will be returned to your ship well before your scheduled departure from port, otherwise they have to get you to the next port at their expense. Vetted and licensed operators also offer more amenities to their packages and they offer a wider range of excursion options over other operators. Bottom line is, the value and guarantees associated with this operator are well worth the slightly higher price of the excursion.
The second option, a third-party supplier, is acceptable because they too must meet the licensing and on-time return to ship guarantees as those who the lines contract directly with. Another advantage is that these suppliers often times bundle some excursions (typically one per port) into a package that has a very affordable price. On the flip side though, these suppliers have very few options that are offered in their portfolio and their excursions often are very bare-bone in their design (they don't include much on the extras or amenities side of things). Our preferred partner for third-party suppliers is Shore Excursions Group and you can book your excursions in advance of your cruise by visiting the page connected to the link below.
Your final option - grabbing a "spur of the moment" tour from one of the many locals who are promoting themselves as tour guides. We DO NOT recommend, nor support these "guides" due to the fact that they have no insurance for the activities that they provide, they have no guarantees of return to port (or anything else for that matter), and their experiences never include anything aside from the ride around the area in their transportation.