Hi Lisa -
The advice that I've been given is that if the horse is hot and the
water is cold, let the horse dring several long sips (4 or 5 long
draughts is plenty), walk the horse for 5 minutes, let it have some
more water, and continue to walk untill the horse cools down completely.
Walking also helps prevent stiff muscles.
It's fine to allow the horse to fill-up on cold water while riding - just
as long as you keep moving after drinking.
On rides, I fill my water buckets before I leave camp in the morning
so that the sun (hopefully) will raise the temperture to a tepid level
by the time I'm back for lunch or the end of the ride. If the water is
warm and my horse has cooled down a bit, I don't worry about how much
he drinks.
Linda