Do I need a passport?

For US citizens, it depends on your itinerary. For "closed-loop" cruises (those that start and end in the exact same US port), you can often sail with just a government-issued photo ID and an official birth certificate. However, carrying a passport is highly recommended in case of an emergency that requires emergency air travel home from a foreign country. 

How do I handle money on board?

Cruises are completely cashless environments. When you board, you will receive a stateroom key card (or use an app) linked to a credit card (which was setup at check-in) to cover all onboard purchases. You do have the option on some cruises to load your car with cash, but this must be done at Guest Services after you board. Typically, you will only need cash or credit cards when you get off the ship in port. 

Do I need to pack my own beach towel?

No, you do not need to bring a beach towel. Major cruise lines provide complimentary beach towels for you to use both on the pool deck and when heading off the ship on shore excursions.
Just remember when taking the towel off the ship that you bring it back, otherwise you may have a charge at the end of your cruise.

Is it mandatory to eat in the main dining hall?

No, it is never mandatory, you have total freedom to skip it. Cruise ships offer multiple included or additional-cost alternatives, letting you choose between formal service, a quick buffet, poolside bites, or room service. 

How long does final debarkation take?

Final debarkation can vary. If you set your luggage out the night before then you will be some of the first passengers off the ship, usually between 7am and 8am.  If you are carrying your luggage off the ship yourself,  passengers are generally required to be completely off by 9:30am to 10:00am, this allows the crew to prepare for the next voyage. 

What happens if I miss my ship at port?

Typically, the ship will sail away without you, and you are responsible for re-joining the ship at it's next port. You can talk to a port agent to see about next steps and gather your belongings.  However there are a couple of exceptions.
If you book an excursion though the cruise line and it is running late, the ship will wait for you.
If there is rough weather, occasionally the ship will delay it's time for leaving port.