I have most of the flats shirts from the various manufacturers. I find the Columbia Bonehead to be too hot, the cotton doesn't breath well and it soaks up sweat (sorry Dan, just my opinion). I live in the Bahamas and just got off the water, it is hot here right now. I was fishing in a Patagonia flats shirt which is pretty good but my favorite by far is my Simms Ultralight shirt. It feels great on and keeps me cool. They are expensive but well worth it!
Good luck & stay cool.