I wouldn't add any water but I would put carrots and celery in the bottom and put the roast on top. The water in the carrots and celery will keep it moist without being watery.
Add no water. It will make it's own juice. A good way to get flavorful juice is to take a pack dry liptons beefy onion or beefy mushroom soup mix and sprinkle on the top. Remember to put the side with the most fat down on the bottom. When done take the juice off the meat and put in a pan bring to boil and let it boil down . Mix 3 tbs corn starch with 1 cup of water and whisp in to boiling mixture slowly until thickened. Makes a great gravy. Also you can put carrots, onions, chunked potatoes and cabbage in the crock pot too and then you have a whole meal. I use baby carrots. Left overs make a great stew.
Just enough to cover the bottom of the pot. Do you have any bouillon. If you are going to be around, check the amount of liquid in the bottom and add a bit more after a couple of hours if necessary and turn down the temp once everything really gets hot. My crock pot is an older version and sometimes it gets too hot and things end up getting scorched. I don't like to put in a lot of water because then it's like boiling the meat rather than slow cooking it.
I would make sure the meat is covered totally. Then cook on high with the lid on but slightly hanging off to release the steam. If you aren't going to be home to watch it cook, ignore all of this.
The great thing about Crock Pots is that you don't need much of anything. A few tablespoons of water is plenty. The slow cooker will create more liquid as the meat cooks.
What kind of roast? Some you do NOT cook in a crock pot. But assuming you have the kind that does, you want the water to come up at least halfway. I like it a smidgen higher. You should also check on it while it cooks in case it evaporates too quickly and you need to add more.
And for that matter, you should use stock & wine as opposed to plain water.
I never use water with my roasts in a crock pot. Put the fat side up, sprinkle some lipton onion soup mix on it, and cook it on high for 4-8 hours. Mine fall apart that way.
It depends on long you want your roast to cook. Too little water will make your roast dry. Too much water will make it watery. I'd start with at least a 1/4 cup of water, then season your roast how you want it. Just remember juices will be coming from the roast also.
I'd start with 1/4 cup of water and keep an eye on things.. if it smells like it's getting dry and too brown.. add another 1/4 cup. It will really depends on the cut of roast you have and how much fat and juices are in it before you start cooking. Don't take off the lid tooo often. It needs that steam to cook right.