There is a rather large liveaboard community at the State run marina in Keehi lagoon. I lived there for several months. It is very run-down and in desperate need of repair. There is also La Marina, which is a private run one in Keehi as well. I heard that, while not allowed, as long as you pay your slip fees, the old woman who owns the marina doesn't care what you do. The private one, Keehi Marine Center, is actually fairly nice has decent showers/liveaboard facilities, but they were really strict on liveaboards and you may or may not be able to get a liveaboard slip.
If you are military, there are several marina's that are fairly nice and cheap. Other than that...there aren't many options other than the ones I listed. The Ala Wai is nearly impossible to get a slip in, it is in very poor condition, as well.
Before you buy a boat, make sure you have a slip. Most, if not all, of these marinas don't allow you to transfer the slip.