The original definition of the metre was that it was one ten-millionth of the distance from the North Pole to the equator, as measured along the great circle that passes through the Paris observatory.
Nowadays all the seven base units of the SI system (the second, metre, kilogram, ampere, kelvin, mole and the candela) are defined in terms of certain constants of physics, which therefore have fixed values even if measurement precision improves. The physical constants involved are
the caesium hyperfine frequency ΔνCs
the speed of light in vacuum c
the Planck constant h
the elementary charge e
the Boltzmann constant k
the Avogadro constant NA, and
the luminous efficacy of a defined visible radiation Kcd
For example, take mass. The base unit is the kilogram (not the gram) and it is defined as follows
It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 x 10–34 when expressed in the unit J s, which is equal to kg m2 s^–1, where the metre and the second are defined in terms of c and ΔνCs.
See www.bipm.org/en/measurement-units/si-base-units for the definition of the others.
The properties of water are not involved.