I'm on the water for between 6-8 hours on a typical kayak-fishing trip.
I used to push it hard when I was actively windsurfing a number of years back. I found that most of my dumb/dangerous moves then were made when I was hungry and/or dehydrated.
Dehydration for most of us can put us in real danger physically and mentally, when we are on the water, yakking, or in the water wading for fishing.
An ER doctor/friend gave a first aid lecture for our fly fishing club at my request. Besides the normal warnings on sun blocking, he spent a lot of time on the dangers of dehydration. Dehydration is the number one cause of falls in the elderly and many others, who are younger.
We in the flyfishing community often wear Gore tex waders, jackets and gloves to keep us dry, which they do. While wearing them, we stay dry, but we lose a lot of our body water via evaporation. This ER doc and other doctors feel that the impact of osmosis where a small body of water/us loses water via evaportation to the bigger body we are in is not understood and underestimated.
Wind is another factor for increased dehydration.
I like to freeze a half full water filled Camel Back bag and add cold water to fill it up before I leave home. If it is really hot, I will freeze a whole bag and get cold water most of the day. My sons and I like the Lipton Teas with sugar for that extra pickup. None of us do very well with artificial sweetners in the heat. We get moon faced and mentally out of it if we take on loads of drinks with artificial sweetners.
The loss of body fluids, also, can take a toll on the necessary minerals in our fluid system. I'm impacted with loss of calcium and need to take calcium tablets and drink liquids with calcium like some chocolate milks. A lot of us are on cardiac drugs and need higher intakes of potassium/K. My wife has this problem and loads up on high K foods before, during and after yakking. She eats slices of avacados and drinks Swiss Miss Dark Cocoa mix with chocolate milk in the mix.
Years ago, there was an article on the need for liquid and food intake during warm to hot weather for fly fishers. The author recommended chilled slices of fruit and veggies, grapes and orange slices. He recommended chilled soups for lunch or after the fly fishing. I have found chilled Progresso Chicken with wild rice soup is great. The can has an easy tab pull top. I drink the liquid first and then spoon out the chicken, rice and vegies. Some of the canned soups have a high level of critter fat in them, and they are not as tasty cold. My wife puts a chilled can of Progresso Basil soup into a metal thermos and drinks it to cool her down. One chilled can for hunger on the water and one in a cooler in the truck works great. Basically there is no handling tricks. I just keep a few cans in the bottom of our frig for fly fishing and yakking during hot days.
We both enjoy chunks of chilled cheese or the good cheese sticks that have been chilled and kept in a small cooler bag.
My doctor ER friend said to reserve the jerky, pretzels, chips and nuts until you were off the water and rehydrated with plenty of liquids. Then, it was a good time to eat these things.
Have cool liquids in a cooler in your vehicle to drink while getting the yaks back on your vehicles and to replinish on the trip home. My wife breaks out a chilled bag of sliced vegies and fruits for the trip home.