I did a mexican cruise once, and didn't care for mexico.
rule #1: smuggle in some booze, drinks are expensive, just don't smuggle in too much or you'l get it confiscated. I just bring 12oz of vodka in a water bottle and i'm good for a 3 day trip... for a 5 day trip see rule #2...
rule #2: at first port, bring two empty water bottles out, and smuggle more vodka back in with them.
i'm a big NCL guy myself, but cruising is cruising. can't remember if carnival has pre-assigned seats for dinner, but if they do, and you don't like your group, fk em, and have them switch you to a new table for the next night, they will do that with no issue.
you might be a big buffet fan, etc, and I am, but we also like to do at least one fancy upcharge dinner, the steakhouses were pretty dam good, but they look at you funny when your ask for A1
I've cruised probably 10-12 times, out of Miami/laud, mostly carribean stuff because mexico was a turnoff for me, I rarely do excursions tho, I found it easier at some places just to do a self guided thing, but a stingray swim might be interesting
take advantage of room service, its usually pretty good and you can get things like tomato juice and orange juice that you can't get at the breakfast buffet. I found it nice to have my coffee and a danish delivered at 8am instead of having to chase it down, then go to the buffet and stuff myself.
if your a drinker, visit the duty free store, they have some good deals, and you get them on the last evening before returning home.
find the cafe that's open till like 2am, there is usually at least one, and go there for burgers/wings late at night.. one of my favorite things.
if you or spouse are on a diet.. 86 the diet for the week, its impossible, just eat and eat and eat.
be nice and get to know your room steward, they will take excellent care of you if they think your a swell guy, lots of ice, towel animals on the bed, turn down room when you leave right away, etc.
look into phone service at the ports, AT&T didn't work for me most times, but other providers work just fine, and no upcharge.
if you plan on internet access, it will be S-L-O-W and expensive. consider just bringing your tablet/laptop to the port and finding wifi for 10 minutes so you can checkup on email.
oh, and good luck in the shower ! i'm only 175 lbs and I barely fit, I have no idea how heavier people bathe in a cruise shower, and i'd stay away from pool/hot tub, overcrowded and gave me heebie-jeebies, but of course i'm a florida guy so pool/hot tubs are not a real big deal to me.
have fun and stay safe in mexico, that whole country scares me !