Been to both a few weeks ago! I went on the Mexican Riviera aboard the Mariner of the Seas (Royal Carribbean). For Mazatlan, we went on this tour and we had to board a bus where the guide would drop us off in various locations. The thing my parents hated is that the tour guide would run off somewhere and not come back until it was time for us to board the bus. So we were forced to look for him every single time. He also call us over back to the bus after spending at the drop off locations for a few minutes.
There was also a performance we were forced to watch too, called Spectaculare! It was supposed to give us an idea about Mexican's heritage and the like, but it really didn't. The show was enjoyable though, but when the guide tried to explain about each skit (how it relates to the "heritage") no one understood it.
He also tried to extend the tour by showing us where they sell shrimp and flowers. It was pretty boring, honestly. I wouldn't recommend you go on the tour. There's one where you can go into a jungle or forest, might be a better idea.
And PUERTO VALLARTA!!! aaaah! Bestest place ever. You're going to love PV!! We had an AWESOME tour guide and he gave us one heck of a tour too. We took around this wealthy neighborhood and saw this mansion and various BEAUTIFUL homes. Heck, PV is a VERY beautiful town. BEAUTIFUL.
And thing about PV, is that tequila is famous there. We had to drive through the mountains to get there, but it was worth it. We also got to visit the location where they make the tequila. Called "Mama Lucia". They also give you samples of each of the five tequilas available. I didn't get to try it though, since I refuse to show them my ID ^^; Anyway my Uncle did buy one though.
And ever had lunch with your Tour Guide? We did! He stopped us by this mexican restaurant which serves of course, fresh authentic mexican food and it was delicious. He told us to eat without him, but we insisted he'd join us. XD It was awesome.
We didn't do much else, though. Only exploring the city. But it was very enjoyable, though! Best of luck to you on your Cruise.