After five days of Florida sun, I adore my Men's Long Sleeve Outrigger Rash Guard (lycra) shirt from Lands' End. At $36 it's not cheap, but I'll wear this one until it falls apart. You can buy similar shirts on Amazon for about $23-$35, I suspect they work just as well. It's surprisingly comfortable when dry in the mid-80s, and when wet it gives me a few comfort degrees in the water and cuts air heat completely.
The shirt had no SPF rating, I've seen them quoted as "18", but this SPF stuff is nonsense. If I'd been wearing SPF 50 sunblock I'd have been disabled by the third day of nonstop sun. I ended up as pale as a dead fish in the area covered by the shirt (I did use sunblock beneath it). I paired the shirt with my ultra-fashionable camping hat -- super broad bill, rear fabric drape from hairline to shirt top. Works fine in the surf, and it's great at keeping beautiful starlets at bay (the latter is a joke, the less fashionable beaches we visit show why American endocrinology has such a great future).
It's true that manly men and Republicans have a hard time wearing these shirts, though I think they may be popular among surfers. Geeks and Democrats, however, should consider them proof of a rock solid ego. Recommended.
Update: For next year's model I want a hoodie. A Lycra hood I could slip on and off would work better underwater than my camping cap ...