They are both in Mexico but are usually visited while on a Caribbean cruise so it would be logical to get it now and be safe from worry. Also, the price will probably keep going up since they are going to be required for any travel soon.
They are considered Caribbean. You will need a passport to return to the U.S. So, you should get one if you want to return home.
Voyager of the Seas 5/03
Celebrity Solstice 2/09
Oasis of the Seas 10/16/10
Get the passport. I am sure they will require it as most ships also go to other ports in the Caribbean and even if they are not scheduled they may be diverted for some reason. A passport should be a required thing to have.