I feel for you, I have the EXACT same thing going on here!
My now-yearling Shetland colt was getting worse last November. My horses are stalled and in shavings, and I was stripping his entire stall daily. Not only was his stall gross and expensive to maintain... he was very bloated all the time, out of condition, and especially lethargic. Overall I considered it unacceptable health!
So, I first figured out exactly how much water he was drinking by shutting off the automatic waterer. He was drinking 9 to 12 gallons per day (this was in the winter time, we don't have extremely cold winters but still, not sweating or anything). I read that the average 1,000 lb horse should drink 12 gallons average... so a 180 pound colt is pretty excessive.
I tried all the "make him less bored" tricks... got a feeder for his hay with VERY small holes, filled his stall with toys (he does LOVE his toys), got a Pony Pop for him, etc. but that didn't deter him. He would guzzle water until he was absolutely bloated.
I had the vet run tests... urine test, then a urine test coupled with a blood test. Nothing pointed to any organ or metabolism problems.
So, the vet said I could safely ration his water. I didn't cut him back ALL the way to where he could have gone down to, but it is rationed back pretty good... he gets two gallons in the morning with his breakfast and two gallons in the evening with his breakfast. This way he has access to water while he's eating to help prevent colic. I do give him all his grain soaked, and when it is extremely hot (over 100 degrees) he can have an extra half gallon or so mid-day to help keep him hydrated. If I could give him smaller rations more often I would, but twice a day when he is fed is the only way I can.
He has been fine like this, he is worked and conditioned just like everyone else. Since he has been rationed, he looks great and he has MORE than enough energy and actually acts like a yearling stallion should!
Every now and again he gets his automatic waterer turned on and he will go right back to drinking all he possibly can. His family does this, so either they learn it from the dam when they are babies or it is something genetic.
My vet said that rationing his water is the healthiest thing for him. Over time they can urinate so much electrolytes out of their system they can crash it. One horse I know of that pees excessively, pees out all of his Potassium so they have to give the horse a postassium supplement every day to try to keep up with replacing it since they don't ration his water.
None of my horses have access to a salt block; my vet does not feel that generally stalled horses need it. He feels only horses out on pasture should have them. I do feed a supplement called Equi-Pride and I think it has salt in it. If you feed any prepared grains or supplements, oftentimes there is enough salt in those already that a horse does not need even more.
I hope any of these experiences can help you... it's not a life-or-death problem but something that can be maintained.
Andrea