The color of any substance is determined by the colors in the spectrum absorbed or reflected by that substance.
If a body absorbs all colors of the visible spectrum, it appears black.
If a body reflects or scatters all the colors of the visible spectrum, it appears white.
If a body reflects only a/few color/s of the visible spectrum by absorbing remaining, it appears in that reflected color/s.
In case of water, its tri-atomic (2 atoms of hydrogen and one atom of oxygen) structure does not absorb any colors from the visible spectrum of the light. However, all the electronic absorption occurs in the ultraviolet region of the electromagnetic spectrum of light, which is not responsible for imparting any color to a substance.
Hence water appears colorless.