Having an endurance horse- keeping them hydrated while traveling is very important. Before a competition or a long trip, I will make sure that all meals I feed have been made into "soup" with as much water as I can get them eat it with. I also will add extra e-lytes to make sure they are drinking. For on the trailer, I feed soaked hay in the haynet. I usually will put the hay in the net, then soak the whole thing for an hour or so before leaving (caution though when it's super hot out that the hay doesn't spoil). I also like to hang a partially full bucket in trailer with them so that they can drink at will and when I stop I'll often float a couple of carrot or apple slices on top so that they have to fish for them.
“While the rest of the species is descended from apes, redheads are descended from cats.” Mark Twain
Endurance rider and vet Susan Garlinghouse has a great 3-article series on her website about "Beating the Metabolic Pull." Article 1 is about hydration. You should check it out. I just do what she says... Go to link and scroll down the list of File Names and you'll see it.
Soupy feed with electrolytes 24-48 hours prior to travel.
Offer water at fuel stops. If horse is reluctant to drink, add some sweet feed to 1/4-full bucket and make "tea." Also, fruit punch Gatorade administered by syringe, poured into bucket, or (if the horse likes it enough) slurped from the bottle.
“A clever person solves a problem. A wise person avoids it.”
? Albert Einstein
I also make horse tea. I use 1 qt of Senior feed and pour water (preferably warm) over it. Sometimes I add electrolytes, depends on the weather and how much they've sweated. The 15 horses at my farm will knock you down to drink it, even when they're traveling.
I make it up beforehand by using a watertight container. (Old Cosequin containers or other supplement buckets work well- the types that have a screw on lid.) I put the Senior in the container with a little water so that it is dissolved when I'm ready to add it to the bucket of water.
I usually make up a big bucket of water with Horse Quencher in it and have the horse drink it before he gets on the trailer. If I had my own trailer and was going to be hauling for a long time, I would take some extra Horse Quencher packets with me and offer my boy some HQ water during the trip. I have yet to meet a horse who will not happily slurp down a whole bucket!
Also, feeding chia seeds helps to keep horses hydrated--they form a sort of "gel" in the digestive tract that aids in hydration. Apparently, a lot of athletes use them for this purpose (in addition to the omega 3 fatty acids, etc.).