Summer is almost here, and I don't know about your neck of the woods but Pennsylvania is getting HOT!
In the hot weather you and your horse need to stay hydrated. For me, the moment I feel any nausea or a headache coming on, I go for my Gatorade. I drink mostly water, but sometimes the best thing for my dehydration is Gatorade. Once you feel thirsty, you are already technically dehydrated, so do yourself a favor and drink lots of water.
I always check my horses hydration level in the heat as well. The best way is to perform a pinch test. What you do is on the side of your horses neck you pull his skin toward you with your thumb and finger. If it snaps back quickly into place, he is OK. If it forms a tent, or slowly goes back into place he is dehydrated.
Your horse should always have access to fresh clean drinking water. I like to scrub out my buckets before each feeding with a brush. I like the kind that are for doing dishes. Nothing special, but they do the trick. Don't leave any slime on his bucket. In the summer, it's nice to put two buckets of water in his stall.
Be sure to check and clean out those outdoor water troughs too. If you see cute little "tadpole" like things swimming around in them, they are probably not going to grow up to be frogs, but disease carrying mosquito's.
If your horse is being worked and is losing fluids through sweating, talk to your vet about electrolytes.
It's like Gatorade for Horses and a good way to supplement them through the summer months.
Enjoy the weather and take care!