Poor a bit of apple juice in their water.
I have that same problem, he refuses to drink it no matter what so what I do is I let the little kids that are hanging around feed him carrots because carrots are 87% and apples are 84%
Electrolytes is the surest way to make a horse drink. Dereck explained very well how to give them and what it is.
To give electrolytes is a good trick. You can put it (in powder) in the food or give it to the horse with a syringe (in his mouth, as a vermifuge). It contains vitamins and many ingredients helping the horse to recover and get back the elements lost when he sweat.
I know some people give their horse electrolytes!
I know some horse like some fruit syrup in the water (apple, grenadine...).
The hay stuff is an ingenious idea! You can also put pieces of fruit or vegetables in the water.
I have already faced this problem. So I have 2 solutions. First I put the hay in the water, so the horse has to drink a little bit if he wants to eat the hay, the hay is wet and gaves a better taste to water! If it doesn't work, I put a salt block in the box (my horse loves it, so it may not work with every horse), which makes him thirsty. He ends up drinking!