The choices are many, Broad brim hats like an Akubra, soft cotton bucket hats or baseball style hats. They are all good functional hats that do the job of protecting you from the sun and sheding water when it rains. Coming from a cattle farming background I wore an Akubra for decades and still have a couple in my wardrobe, they definitely give you plenty of protection from the sun and nothing sheds water like the Akubra so a good choice. The bucket hat made of soft cotton is comfortable and protects from the sun and keeps you dry in light rain but they don't have a good peak to shade your eyes from the sun this is important when polaroiding for fish. The base ball cap style I find the best it's light comfortable. protects your head and eyes from the sun and offers some protection in light rain. It's major draw back is it offers no sun protection for your ears, as I always wear a neck sock to protect me from the sun and reflection from the water I have no problem with this style of hat. Be sure that the underside of the peak on your cap is black or a dark colour to minimise reflection from the water. This is very important when polarioding for fish.
So do your homework and choose wisely.