Converting mm Hg to mm H2O
You CAN use R=8.31 but it has the (standard) unit of (m3 Pa K−1 mol−1), simply convert your volume from liter (aka dm3) to meter cube (m3) and the pressure will be in Pascal (Pa)

If you use R=62.36, which has a unit of (L mmHg K−1   mol−1), as long as your   volume is in liter, the pressure will be in mmHg
But the question ask for mmH2O.

Pressure is simply force on an area (F/A), weight on an area (mg/A), density * volume on a area (rho*V /A) = density * height (rho*h)

Thus P = density (Hg) * h (Hg) = density (water) * h (water)

h (water) =  (13.6 / 1.00) * h(Hg)