I put a boat seat umbrella mount on the backs of my pedastal seats in my jonboat. My wife's idea; I chuckled the first time she used one, thought it looked a smidge silly, but it didn't take too much hot sun beating down on me that I got jealous and bought one for my seat, too. Had to learn to cast sidearm but the shade from the umbrella in the mid day really helps. Gets a bit tough if there is a good breeze, but then again if there is a breeze it's not usually as uncomfortable.
Other tactic is that I will sometimes take off my shirt and soak it in the water, wring it out so it's just damp, and put it back on. The cooling effect is surprisingly helpful and can make a big difference in comfort.
Similar to Sam's idea. I wear all the sun protective gear I can, and like to put some cold water on my shirts and buffs to cool down. One trick that helps me a lot. I take some ice cubes out of cooler and put in my hat. The cooling effect lasts longer, and if any happens to run down to my clothing, so much the better.
My boat has a hose on the livewell pumpout for filling a weigh-in bag, if there are no fish in there I will fill it up and use that hose to cool me off. I have also been known to take a break under a big shade tree or pull into and empty covered boat slip and have a snack or cold drink.
If I know it is going to be hot, I make sure to start hydrating my body a few days ahead of time and always carry plenty to drink.
I fish 5-6 days a wk most all yr. I've found that those old farmers had the rite idea bout bein out in the sun all day. I wear long sleeve shirts an long pants all yr. I use Cabela's Guidewear an can fish comfortably in 95* heat. If the sun don't hit ya skin ya feel cooler. an just a little breeze will cool you while wearin these type clothes. I also have an advantage over the 'yaks too, my boat goes over 30mph an that makes ya feel like ya in the air conditioning! I fish a spot for 15-30 mins, then run down the lake for 3-4 mins an yur dry agin, stop an fish up another sweat, then turn on the "air conditioner" agin.
Long sleeve white SPF Tee with a buff & good wide brimmed hat. Frog Tog Chilly Pad in the drink cooler, wrap that on my neck when I get hot (good ol' towell works just fine too). Drink a bottle of water every hour or every half hour. Shorten your day if you have to. Heat exhaustion can be very dangerous.
I will wear a quality name brand light fishing shirt such as a Columbia or Under Armor along with the light Columbia pants with the zip away legs. These keep you cool and dry quickly as well. I also wear a somewhat goofy but protective hat with the shade on the back. Drink lots of water to stay hydrated and get off the water (even temporarily) if you are feeling the effects of the heat. Those cooling kerchief things you wrap around your neck work well too. And parking the vessel and doing a bit of wade fishing helps as well!