The relationship would be inversely proportional, meaning that as the volume increases, the pressure decreases, and the opposite is true as well, as the pressure increases, the volume decreases.
The average mass of one H2O molecule is 18.02 amu. The number of atoms is an exact number, the number of mole is an exact number; they do not affect the number of significant figures. The average mass of one mole of H2O is 18.02 grams.