What happens if you use water instead of mercury in determination of atmospheric pressure?

I'm not sure if this answer is the kind of answer you're looking for, but basically, you would have to use a much larger amount of water than Mercury to get an accurate reading of atmospheric pressure. This is because water is not nearly as dense as mercury, so it takes much less atmospheric pressure to raise the water up the tube in a water barometer. This results in water rising much more easily than mercury. Obviously, then, a water barometer would have to be much larger than a mercury one. Since mercury is more dense than water, mercury barometers are much smaller. So, if you put water in a mercury barometer instead of mercury, the reading you get would be way too high.

Air pressure at sea level is about 14.7 pounds per square inch (psi) and can push a column of water almost 33 feet high, but it can only push mercury 30 inches high. Roughly, ever inch of mercury equals a foot of water.