Often copied, never quite equaled, the Tilley Hat is the most all-purpose chapeau I know. Its broad brim keeps the sun or light rain off, the bit of foam in the flat crown pads the skull against light whacks and keeps the hat floating in water, and the double strap defeats all wind. (Use just the rear strap behind your head to hold the hat in all but strong winds; the front strap under the chin is effective but dorky looking.) The Canadian behind the Tilley line has generated a humorous but effective fetishism around his durable hats. With one or both side brims snapped up to the crown you get a rakish look which also stiffens the front brim against wind. The Tilley packs well and does last for many years; however, a lot of hot weather use will stain it incurably with sweat. You may not mind.
[Please see the more recently-reviewed Tilley Airflow.]