I've been on 2 cruises and they were the best vacations! I'd go on another in a heartbeat! The food was excellent, there was plenty of entertainment, and shopping in the different ports was a blast too! The first cruise was in August and the second was in February. People were still laying out in February but it was too chilly for me! I never had a problem getting a chair on the deck to lounge in, but that's not my favorite thing to do anyway. I guess it depends on the size of the ship and when you go. They've changed the dining on most ships now so it's not quite so formal. Dressing nicely for dinner never bothered me and the seating arrangements were fine too (unlike that commercial on TV lately). DH and I ended up taking an extra excursion with a couple who sat at our table. Just remember that most beverages (soda and alcohol) are extra. Both cruises I took were Carnival cruises and they had a restaurant you pretty much just walked in and got your food buffet style. There you could get juices and what not without extra charge. We usually ate breakfast there because we liked to sleep in. Plus there you could eat out on the deck or inside.
Plan some extra cash for excursions and things on shore. We needed to take a cab both times we were in Mexico. During the first cruise I was still in high school and went with a group. Our routine got to be relaxing in the deck top hot tub after dinner, and then heading to the club (they had an under 21 side as well as a teen club, which we never went into) to dance for a while. And then sometimes after that we'd go back to our room and order room service. (Wish I could still eat like that and not gain weight! Guess we were really active then though!)
Dh and I looked into a few different vacations and it really worked out that the cruise was cheaper (the February one, which we only booked about six weeks in advance, including our airfare). We'd looked into flying into Key West and staying a week. The cruise was cheaper by far, especially when you take into account the food and entertainment. One tip I do have is to pay when you make your reservation to take the cruise lines shuttle from the airport to the ship and back. It saves time and headaches with transferring your luggage. DH and I were able to pay for that when we got to the airport, but it's just easier to make that arrangement ahead of time.
I've not been to Puerto Rico yet, but my parents have since that is where my step-family is from. They love it there. We also have a friend who works there every winter for a few months. There is plenty to do and see. Mom said the biggest surprise to her was that some of the beaches weren't as clean as she'd have expected, but I don't know which ones they went to. It may have not been in the more tourist type areas since they were all over the island visiting with family.
If you have any other questions about cruising just PM me, I'm happy to answer them!
And have fun!!