Water is key to mankind settling the Solar System. It can be used as water, or broken down into hydrogen and oxygen, it can provide fuel for rockets and air to breathe.
We now know water ice exists throughout the Solar System, from Mercury and Mars all the way into comets in the Oort Cloud. The largest ocean under the Sun may exist under the water ice shell of Jupiter's moon Europa, not on Earth.
Finding water won't be a problem. As we move out, we will likely move from water to water, like desert caravans move from oasis to oasis. The proper analogy of an interplanetary spaceship, then, may be less the sailing ship and more the camel.