On the cot question. After a bunch of cot purchases, on the advice of a seasoned Grand Canyon boater, I purchased a roll a cot. Bomber design and construction. One of if not the best purchases ever of camping gear. As mentioned previously, in the heat, sleep on the mesh with a wet cotton sheet for coolness. If it is cold, add a paco pad or a thermarest. I have and use either Paco or Thermarest depending on the trip. In a raft I use the Paco, in a canoe it is usually the Thermarest.
My first Grand Canyon trip was in mid august. When it rained it was cool sleeping for sure. We just pulled a tarp over head and enjoyed the cool time. Most nights I slept on a pad on the river's damp sand with feet close to or in the water. Be prepared to move up or down the bank depending on the "tide". If beach is short and steep, just get close as it makes sense, normally there is cool air within 4 or 5 feet of the water level.
Plenty of great tips already posted on other things to bring to make for a more comfortable trip.
Be prepared for the blistering hot sun during daytime lunch stops. The hot sun can actually burn bare feet. If in kayak or raft, keep teva's binered in close by, maybe light sox for the feet's sun protection. A nice wide brim cotton hat (dip it in water for more coolness) is a must. Same procedure for a light weight cotton long sleeve shirt. I like the fishermen's models as they have plenty of pockets for sun screen, lip protection, sun glasses, hand/foot salves etc.
Be aware that the August sun seems to me to be extra powerful. Just a few extra minutes of bare skin (or even wet or dry cotton garments) can leave most of us with a painful burn that makes the rest of the trip very uncomfortable. Carry plenty of aloe plus sun screen.