This method makes the BEST roasted prime rib. It never fails to produce a tender/juicy prime rib for me. Don't worry, the salt mixture will not affect the taste. It's purpose is to seal in all the flavor and juices.
Slowly add a little of the water to the Kosher salt and stir; then add a little more water and stir. Continue until the salt becomes the consistency of "snow" so that you can "pack" it on the roast and it sticks like snow (Sometimes you may need a little more or less of the water) -- YOU DO NOT WANT TO DISSOLVE THE SALT.
Pat some of the salt mixture in a foil-lined pan in a rectangle big enough to put the roast on.
Place the roast on the salt and pat the remaining salt mixture on the roast, covering completely.
Inset a meat thermometer.
Bake at 425 degrees for about 2 hours.
(140 degrees for rare, 160 for medium).
Remove the roast when the thermometer registers 5 degrees BELOW desired doneness.