If you are very concerned about the rain and want to stay as dry as possible, you might want to invest in a pair of Frogg Toggs. They are very lightweight, easily packed, and will keep you bone dry. Plus they breath very well, and you won't get soaked with sweat. My wife and I keep our sets in a vaccuum sealed pack in our park bag so as not to take up space. In the event of rain, we find a quick shelter, throw them on, and "just keep swimming."
Something else to consider is shoes. No one likes to walk around with wet shoes and socks. Something we do is pack a pair of Croc's Beach shoes with our Toggs. That way when the rain is over, we just put dry shoes and socks on. You can also buy shoes that are GoreTex (pricey) that will keep your feet dry. I have a pair of New Balance Trail shoes that are super comfortable that work very well. Went last year during the week before Christmas. Rained everyday (or so it seemed!), and my feet stayed dry! There is a North Face Outlet in Orlando, and you can pick up a GOOD pair of outdoor gore tex shoes for about 1/3 - 1/2 what you would normally pay! As for the Frogg Toggs the best places I have found have been Ebay, Academy Sports, and Bass Pro Shops.
Don't let the rain keep you away!! Rainy days are some of the best times to visit the parks, as the crowds typically dwindle, and you can do and see a lot more than on days with no rain.