Yes. While you may choose to stay on board, the cruise line may not let you book without a passport. In the event of an emergency (illness, running aground, ship fire) evacuation at any port may be necessary. Disimbarking and embarking would become an issue.
Yup
Check with Carnival's website. If I remember correctly, you can use either a passport or your driver's license and a birth certificate. I've taken three cruises with Carnival and I've always used a passport.
Yes, you need a passport. If you are leaving the USA and going to Mexico then you need a passport.
Yes if you are leaving England then you must have a passport.
Try leaving the country
Whenever leaving the US (and returning) it's always best to have your passport readily available.
If leaving from Florida, Jamaica is accessed by traversing across the Caribbean Sea. If leaving from Texas/Louisiana, you traverse across the Gulf of Mexico and then the Caribbean Sea.
Only if leaving country
If you are leaving the country you live in, yes.
No. You don't normally need a passport except when entering or leaving a country (There may be countries that require internal passports, but if you live in one, you already know about it.) If you are traveling outside your home country and HAVE a passport, you should always keep it with you in case you need to prove your citizenship, though normally this isn't an issue.
No , you do not need a passport unless you are leaving the US.
No. The only time a passport is required is when you are leaving U.S. borders.