When proofing a room, it is best to acclimate him to the room by spending time with him in it. Provide some unbreakable toys for him, ones you know he won't chew into bits or choke on. Check the room so there is nothing he can hurt himself on if he is left alone with the door closed. Watch that there is nothing he can tip over or actively damage before you leave him there.
Pay special attention to exposed cords, electrical wires, and window treatments. Make sure there are no window dressing cords he can entangle himself with; my own dog almost killed herself by getting caught in the window blinds cords while trying to get a better view when I wasn't home. I thought I resolved that by pulling the blinds up, but she went over and chewed through the window cord. I repaired both and now pull the blinds up even with the cord, so neither are in her immediate view or access.
It pays to be proactive in your maintenance of the area which he will be roaming, unsupervised, so keep the area clean of trash bins, especially plastic-lined ones, and detritus-filled containers, such as ashtrays. Keep an active eye out for things that your dog might negatively access in the environment and proactively remedy them.
Providing access to water prior to leaving, then taking it away for up to 8 hours is not cruel behavior unless the dog requires it for medical needs. It will be a learning experience for the dog to know that he needs to drink prior to your departure, and ensuring you provide water to him when you arrive is fine as long as you are diligent. If your dog is fine with water being around, put it in a very heavy, stable bowl he won't be able to tip over.
Make sure you let your dog out to relieve himself before you put him away, and let him go out again as soon as you get back. If your dog has issues with retention, you can consider using a urination "wee wee" pad in an area the dog can access while you are gone (a shower floor, for example). The pads usually have attractants in them and should contain the mess, though the cheaper urination pads tend to be flimsy and rip easily.
If your dog is especially anxious, you can try a Thundershirt, but only put it on immediately before you leave and take it off as soon as you get home. Leaving it on for longer periods can have a dog get used to the shirt and it will become less effective.