How high in (mmH2O) is the column of water in the tube?
Consider the air pressure in mm of mercury given. Is water more or less dense? Would an amount of water of the same mass as the mercury in the column take up more or less room?

So what would you need to do to change the 735 mm of mercury to an equivalent mass of water, given the densities cited in the problem?

The diameter of the column is red herring; don't go off on the kipper.