Have a passport! Even if the cruise is a closed loop (starts and ends in U.S.), you cannot get back to the U.S. easily if you have an issue in another country.
Take the first hour or so you are on board and just wander around the ship. It's okay if you get lost, but you need to check out all of the places onboard – and now is a good time to snap photos of yourselves in various spots onboard before you get too busy later in the trip and forget.
Plan on doing the excursions. That's the best way to maximize your enjoyment of the ports the ship sails into. The smaller the ship, the more focus there will be on the destination. The larger the ship, the more focus will be on the activities on board the ship. The more ports, the better opportunity to experience the destination. Fewer ports means more shipboard time.
Prepaid gratuities make your time at sea totally relaxed with no additional obligation.
The ship is also a destination and should be enjoyed just the same. Take advantage of all things fun!
If the port of embarkation/disembarkation is in your bucket list, why not stay for a few days before or after the cruise and explore the area?
Plan ahead and be flexible, you can book cruises 12+ months in advance.
Don't spend too much time in your room. Get out and enjoy what the ship, scenery and what the cruise director has to offer. Live it up! You'll never see these people again!
Always take a carry on with a swimsuit or extra change of clothes, due to the possible delay with your luggage getting to your cabin.
Check your passport expiration immediately – it needs to be valid for 6 months after you return. If you're using a birth certificate and government issued photo ID, make sure they have a raised seal – not a hospital birth certificate. Always pack those documents in your carry-on luggage.