Wiki:
"The surface of Ceres is probably made of a mixture of water ice and various hydrated minerals like carbonates and clays.[8] Ceres appears to be differentiated into a rocky core and ice mantle.[4] It may harbour an ocean of liquid water, which makes it a target of current searches for extraterrestrial life.[16] Ceres may be surrounded by a tenuous atmosphere containing water vapour."
http://en.wikipedia.org/wiki/Ceres_(dwarf_planet)

You Tube: Water in Ceres, Mars and Moon - Space colonization

"If Ceres' mantle accounts for 25 percent of the asteroid's mass, that would translate to an upper limit of 200 million cubic kilometers of water, Parker said."
http://www.space.com/scienceastronomy/050907_ceres_planet.html

So enough water for 200 million years:)
Though Ceres is probably too big to put in GEO due to it's gravity affects. It might work at Sun/Earth L-1 [though there is a mass limit at L-1 - don't remember what it is].

But if you could put the biggest mass object possible at GEO the gravity from such an object could reduce the mass needed for a Space elevator. Or one could use material such as steel instead the proposed super material which has extremely high strength to weight ratio [e.g. nano-carbon tubes]. At least for most of it's length.
I believe I did post with some rough calculation for such a space elevator using a body with 1/100th of Earth's gravity [a rock around the size of Eros].

I think the most perfect rock I know of for a space elevator would be the Dog-Bone Shaped Asteroid:
http://apod.nasa.gov/apod/ap000510.html
Because it has a high density, is the right shape, and you could mine for material to be used for space elevator.

But I can't say in general I'm a fan of space elevators- unless it's a matter of Spacers building it to reach down to Earth- something won't even begin to happen in next couple of decades.