Go with Gonzo's suggestions--and warnings! As a whitewater guide for over 20 years, I can tell you that he is right on. It is a rare river where you actually have to worry about getting too hot when you're on the water, so don't hesitate to wear long sleeves and pants to keep the sun off. But do stay away from cotton. If it is REALLY hot, and the water is warm, a T-shirt will probably be OK, but the usual river-runner's motto is simple: COTTON KILLS!
So for sun protection, wear a loose nylon or other synthetic windbreaker and pants, and a large-brimmed hat (if you don't wear a helmet). There are lots of good sunscreens out there that are fairly waterproof. Apply liberally to your face and hands, and carry the bottle/tube in your pocket to reapply regularly. If there is skin exposed on your feet (ie. if you wear the standard river-guide's "Teva"-type sandals), your feet will be particularly vulnerable to the sun. They will be wet a lot, so sunscreen won't stay on well. And they get the sun directly. If you're concerned, wear wetsuit booties, or canvas sneakers, probably without socks. Sneakers will give you better protection from rocks and other hazards, both on shore and if you should happen to go for a swim.
If it's cooler, you may want a wetsuit, which the outfitter probably rents for a reasonable price. That will take care of the sun problem! If the water is fairly warm, but it's a chilly day, a layer of synthetic fleece covered by a windbreaker or a nylon rain suit might be in order. Avoid cotton!
If this is your first raft trip, it is natural to be a little anxious. You'll quickly figure it out. If it's a summer trip, with warm air and water, you really have little to worry about as far as staying warm and comfortable. Plan to get wet. Listen carefully to the guide's instructions, note how he/she dresses, and don't hesitate to ask specific questions.