How old is your horse, OP? Is this a new thing that has been going on with your mare? The reason I ask, is because excessive urination can signal a health problem, such as Cushings Disease. Cushing's disease usually affects older horses, especially those in late teens to early 20's.
Just to give you some insight, I too have a horse who was an excessive peer and drinker. I had him checked by the vet, who at the time said he is perfectly fine, he just drinks out of boredom. I tried giving him toys to play with, limiting his water intake to 15 gallons a day per the vet, nothing worked. I had always suspected he may have Cushings, but my vet said he tests normal. So finally come spring this year, my gelding showed more positive symptoms of Cushings, such as slow shed out of coat, loss of muscle on top line, ect. My vet then diagnosed him with Cushing's.
A lot of the time, a horse will only start out with one or two symptoms of Cushing's, and you wouldn't really expect it, and even if you test them, they may not have a positive reading. You know you have a Cushings horse when you start seeing multiple things going on all at once it seems like, such as the heavy hair coat, loss of muscle, laminitis, abscesses, the list goes on.
I don't want to scare you, but just want you to be aware of what could be expected. You may want to talk to your vet.