It really depends on what you will be doing but I suggest that you bring a waterproof jacket, not water resistant. Many water resistant jackets end up becoming soaked in a heavy rain. A poncho can be used and does cover your lower area when sitting. I do suggest that it is durable. The cheap ones tear very easily.
I brought my Columbia, waterproof outerwear. It is lightweight, breathable and packed easily. It came in very handy. While everyone else was inside looking at whales through fogged up windows I was outside with a perfect view.
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I am a clear advocate of using what you have and not spending big money on stuff you won't use much again. Walmart has some nice long ponchos, that are heavy plastic for under $10, but the cheap ones, if careful work fine. If you are planning on being out on deck extensively in all weather- then the rain coats are recommended, since the ponchos blow over your head. In ports, the ponchos work. More important if you do hit some rain is to have 2 pair of walking shoes. To dry them out more quickly, insert hand towels loosely into each shoe. The wicking action speeds up the drying.
Get some "Camp Dry" from Kmart or Walmart, found in the camping section. Spray your water resistant jacket (and shoes) with that, in order to water proof it just like you would to water proof your tent. That way you don't have to go buy a new water proof jacket. Add an inexpensive rain poncho for any major down pours. That is what I am going to do when we go there this June.
Independance of the Seas
Monarch of the Seas, October, 2007
Serenade of the Seas June, 2006