Depending on your ports, it will likely range from the low 80s to high 80s by day and in the 70s at night. I doubt it ever gets cool, but if there's a breeze, you might want to have a light jacket or wrap for evening. You'll probably need it more for the air conditioning!
Check the specifics on your cruise page here on carnival.com. You should be able to find normal temps for all your ports.
Bon Voyage!