Just did our first cruise to Mexico, and it in no way compares to traveling on the ground. The only authentic experience we had was one I booked on my own, a food tour in Puerto Villarta a 30 min drive from the cruise port, experiencing local food and drinks, taco stands, agua fresca on the side of the road listening to the story of the families that operate each place we went, walking the streets, interacting with the locals. Every other place felt fabricated and fake, with me watching the clock all day long. Our guide told us about a family that makes it's own chocolate and coffee, but the owner said don't bring the cruise people in too many of them, so after the walking tour I went there on my own and bought coffee to bring home.
The cruise is about the cruise not the ports. You get a small taste of each port and only see the touristy part of it, that is not traveling to me, it's like a rat race, rushing to see (not experience) the thing you're supposed to see being shuttled around and everyone knows you're from "the cruise" and you're treated that way. We did enjoy the service, the on-board activities, but we had 2 days at sea that I felt were wasted, I would rather have been on the ground doing something.
You will get a much more authentic experience with a RS tour on on your own.