One of the best hot weather hats I own is one purchased many, many years ago in Florida. Nearly every flats guide in Florida owns one, and at that time they were known as a Florida Guide Hat. Now they're known as an updowner hat, they've been modified a bit and available, in regular hat sizes, at Bass Pro Shops.
There are several variations of this design, some of which come with a lanyard that clips to your shirt collar to prevent you from loosing the hat if it blows off. The back cover of some models, including the one I own, folds up and is held in place with Velcro.
I also own the West Marine hat pictured in some of the above posts and it works great. It's not a OSFA hat and from what I recall, comes in sizes S,M,L,XL.