The oblateness of Ceres seems to indicate water. Moreover water geysers were recently detected by Herschel Space telescope. Hopefully we'll learn more when the Dawn spacecraft arrives February of 2015.
Planetary chauvinists like to point out mass of the Main Belt is small compared to Earth or Mars. But most of a planet's mass is inaccessible. As we burrow down, pressure and temperature increases. Only a thin shell of a planet's surface mass can be reached. A planet's potential real estate and resources are better gauged by surface area than volume.
And the total surface area of the asteroids far exceeds surface area of large moons and rocky planets.
Moreover, the entire volume of an asteroid is accessible.
According to Seligman a body's internal pressure can be approximated by 3/8 π G gR2(1-(r/R)2). Where R is distance from surface to center and r is distance from center.
If my arithmetic's correct, the pressure of Ceres' center is about the same as the rock 2.7 km below earth's surface, about 1400 atmospheres. And the South African TauTona mine goes as deep as 3.9 km underground. Thus it's possible to tunnel clear through Ceres. Of course the volume of smaller asteroids would be even more accessible.
Planetary chauvinists like to say planets can be terraformed. They like to imagine Martians walking in shirt sleeves under the Martian sky, breathing Martian air. In my opinion terraforming is impractical and a waste of resources. I believe living places beyond earth will be wholly enclosed, artificial environments. This will be true regardless whether the habs are asteroidal, lunar or martian.
Another argument for planets is the need for gravity to stay healthy. In spinning habs, inertia in a rotating frame (aka centrifugal force) can provide acceleration to prevent bone and muscle atrophy as well as drain sinuses, enable flush toilets, showers etc.
The asteroids' very shallow gravity wells make beanstalks (aka Clarke Towers) more doable. If the asteroid has a healthy angular velocity $\omega$, even more so. Ceres' sidereal day is about 9 hours, an $\omega$ more than double, almost triple that of Mars and Earth. A Ceres beanstalk could be as short as 706 kilometers and taper ratio using an ordinary substance like Kevlar would only be 1.02. A far, far less challenging project than an earth or even a Mars elevator. A Vesta elevator could be as short as 265 kilometers with a Kevlar taper ratio of 1.01. I believe asteroidal elevators could greatly reduce reaction mass needed for delta V. Thus the mass fractions mandated by Tsiolkovky's rocket equation become less of a challenge. Travel throughout the main belt could be less of a problem than travel between planets.
So my answer is yes. Yes, colonizing Ceres is feasible. As well as colonizing much of the Main Belt.