The dry season is December - May, although the temperature is pretty constant throughout the year. We cruised there this year with Princess, departing 20 April, returning 13 May and the weather was very good. The itinerary was fantastic - leaving from Los Angeles and visiting Mexico, Costa Rica, Panama, Nicaragua and Guatemala. However, be prepared for the poverty in Central America, particularly in Nicaragua. The Panama Canal must be seen to be believed and was particularly enjoyed by my husband who is an engineer. Enjoy your cruise.
We are booked on a Panama Canal cruise for the 3rd Jan 09' with Celebrity on the Mercury, we have been given information on this site that this a a good time to go, we are looking forward to it, so I hope you find something that suits you.