I find the ideal sun protection in really hot weather is an old 100% cotton dress shirt (preferably a size or two too big). It'll reflect some of that heat and light away, and the loose fit keeps it from sticking and chafing. If it's really hot, a hat-full of river water dumped on you will keep the shirt cool for quite a while and rinse some of the salt from your perspiration away. It helps keep the blackflies and mosquitoes at bay too, as the flies can't bite through it, and the mosquitoes tend to be held away from your skin somewhat by the loose fit.
Don't worry- your friends usually stop laughing at you by the second day when they're all itchy and sweaty, while you're still cool and comfortable
The key is tight woven cotton. I also just ordered a cotton Angler shirt from Cabela's. Most outdoor shirts are nylon in the stores. I have had better luck finding travel shirts in cotton at Bass Pro in Florida
Here is the Cotton angler shirt but their pictures arent working! I had to order a guys shirt. Women apparantly do not get functional shirts with lots of pockets, but oversize works best anyway and the colors aren't too manly.
I swear by Ex Officio long-sleeved shirts. Feel like cotton but quicker-drying and lighter. Finding ones with good-sized pockets has gotten tougher -- the women's ones used to have the same pockets as the mens, which they seem to have gotten rid of in pursuit of style, boo. The men's ones are comfy, too -- they're really not strongly shaped garments, so as long as the sleeves aren't comically/uncomfortably long, the men's ones should work too.