We have a gelding with this problem, He will not drink out of stream, a dirty bucket, pond or another horses bucket. He is also picky about the temperature of the water and refuses to drink if it’s too cold or to warm. After much aggravation we realized that he is a hardy eater, and likes his grain and beet pulp, I figured out that if I take his feed bucket with me when we travel I can toss part of his grain and beet pulp ration in the bottom of the bucket and fill it with water and he swills it right down to get to his grain. also a little molasses sometimes works but not always it depends on the day. Be sure you start doing this at home before you travel, that way he is used to it. We also add a little smartlytes to the water to encourage healthy water intake. Have you tried carrying a few gallons of water from home?