Evaporative cooling. Dip your hat and sleeves in the water. I also take a thin cotton dish towel, wet it and put around my neck. If you are in a float tube, then roll the waders down and splash water on your thighs. I keep an extra empty bottle on my float tube for pouring water on my thighs. Works as a pee bottle too (rinse before using to pour water). I also wear a thin, light colored (even white), UPF50 long sleeve shirt. My hat is light colored too. I would wear white waders if I could get them.
You can buy one of these but I am not convinced it works better than a dish towel:
https://www.google.com/search?q=evapora ... b&ie=UTF-8
I use evaporative cooling for my water bottles (2 of the 28 ounce size) on the float tube too. I have bicycle water bottle cages that put the bottom of the bottles in the water and then I have an old sock over the water bottle to wick water up the sides of the bottle. Keeps it about 10-20F cooler than the lake water temperature. If you are in a boat, then bring lots of ice water in a thermos. Have something salty to eat to replace the salt lost in sweating (beef jerky, pretzels or sesame sticks work). Beer may taste great but it is a bad choice for being in the hot sun all day.
If I was shore fishing, I would use a Camelbak (or similar hydration pack) with water and lots of ice cubes. Getting your feet wet helps a lot too. If I am going to get my feet wet, I wear double layer socks so I don't get blisters if I have to hike any distance.
https://www.wrightsock.com/collections/ ... AQQAvD_BwE
I have fished Barrett when it is 103F.