My fiance used to do this run as an assistant cruise director. She says it is very hot at this time of year as it's right on the equator. You should pack lots of light clothing, but dont forget formal wear as well. Dont worry about not having enough clothes to last the entire cruise as there are washing facillities and services onboard.
Royal Caribbean and Celebrity do not have self service launderettes and their laundry service is quite expensive. Sometimes on longer trips (18 days probably would count) they do a special deal of fill the laundry bag in the cabin for a set amount. This is quite a good deal usually but don't count on getting it. Did this trip in 1997 and after another 17 cruises still count it as one of the best for the itinerary.
On the 18 day cruise on the rapsody of the sea to the Panama canal. How many formal and Dress suit evenings are there please, and casual clothing eve's too.