Indeed, the Earth has much less water than the average person probably believes, given the fact that the Earth is sometimes referred to as a "water world". Also, we are often being told that other celestial bodies (such as the Moon and Venus) are uninhabitable partly because they lack water.
So the Earth doesn't have much water, comparatively speaking, but we have it where it counts - on the interface of things. Water is "the skin of the world", sitting right between our protective atmosphere and the still roiling and active interior of our planet's crust. The water of the Earth is continually fed energy and substances from above and below. It is an incredibly dynamic substance here on our planet, given all the input it continually receives.
And interestingly, the most "alive" parts of the water of the Earth are the shallow parts where water meets land. Here is where you find the most activity and the greatest number of biological species, as well as - or so I think - the greatest biomass.
Indeed, water as an interface between active layers above and below may be the key to biological life. A ball of ice floating in space may be rich in H2O indeed, but it may be completely dead.