Just to make sure everyone knows there is a difference between a Real ID and Enhanced ID. Gold Star equals Real ID and allows you to board domestic flights, enter some federal buildings and Military Bases. If you do not have a passport (or passport card) you will still need a birth certificate to cruise with a Real ID (Gold Star). Every state offers a real ID. These do not prove citizenship.
The Enhanced Driver's Licence Has a hologram American Flag on it and is only offered by Five States (New York, Vermont, Washington State, Michigan, and Minnesota) for an additional. These are designed to speed land and sea border crossings and prove identity and citizenship. They are a combination of a Real ID Driver's Licence and a United States Passport CARD (not book). They only allow travel on domestic flights and across land a sea borders. They will not work for international flights.
If you are on a Closed Loop Cruise (Departing and Returning to the same US Port) a real ID and Birth Certificate are all you need an enhanced driver's license (From one of the five listed States) or US Passport Card are also sufficient. If an emergency should happen and you are traveling with only the above ID's additional steps will be needed to leave a cruise early for an emergency. The cruise line will most likely assist. As always, barring any unseen needs for Visa's, an US Passport Book is also allowed and acts as a fail safe against any possible travel issues.
The one exception to cruising on a closed loop cruise without a passport from the United States that I am aware of is if the cruise visits some French Caribbean islands (Such as Guadeloupe or Martinique). These require a US passport because of how flight to and from France are handled as domestic and do not clear customs upon arrival to France.