What can I expect when taking a cruise from the USA to Mexico (and others)?

This is the first time my wife and I have ever taken a cruise, and the process and requirements for this trip are more then a little confusing.
We are departing from Tampa FL, traveling to Mexico and Grand Cayman, then returning to Tampa FL.
It's unclear if we need a passport card, passport book, or nothing. Should be get local currency before we leave? Where would we get that currency? How commonly are credit cards accepted? Can we just stick with those?
In short, what can we expect, process and "money" wise on this trip. I wouldn't think it would be too complicated, but the cruise site (and to some degree the passport site) are very confusing.
PS. We are both US Citizens
Best Answer
To add extra detail to Dorothy's answer:
We were required to provide a valid passport in order to board. We had priority boarding, so I don't know if this was a requirement of that (as in if we didn't have a passport we would have had to go to the normal lines), but not having a passport that was valid for at least six months after the return date was not an option.
In both ports we were told to provide only the ship board ID and a valid government issue ID (Drivers licence). We were advised to leave the passport in a safe on the boat.
When returning to the boat, the ports (Mexico and Cayman islands) didn't even check the IDs. The cruise people checked the ship board ID as you got on and off the ship. They didn't check the IDs either (but photos were embedded in the ship board IDs).
As to the question of money. Cash was accepted everywhere. Credit cards were not. Some places took cards, some didn't. But cash was accepted everywhere. As an aside, be really didn't spend any money. By purchasing ship "excursions" every thing was paid for or charged to our ship board account. I would still advise taking some cash, but not much.
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