If you like to swim, a California apartment complex with a pool may seem like a great deal. It's not so great if the landlord shuts down the pool or if the surface is coated with scum. California law requires the landlord to deliver any amenities he promised you and to keep your living area, including the pool, fit for habitation.
California is one of many states with an implied warranty of habitability: The landlord must keep your rental in livable condition, even if the lease doesn't say so. That includes areas around your rental such as the walkways and the pool. Habitability isn't an exact standard: Draining the pool dry wouldn't count, but if it's coated in scum or has dead rats floating in it, that might constitute an unlivable condition by California rules.
If your lease or rental agreement gives you the right to use the pool, the landlord cannot legally keep you from swimming or leave the pool unusable. Even if your agreement was oral or in the apartment brochures, rather than in the lease, you could still claim the landlord cheated you. It may be harder to prove with an oral agreement, though. A temporarily unusable pool probably doesn't give you grounds for legal action, however, and the landlord does have the right to renegotiate terms when you renew the lease.
If you believe your landlord isn't honoring the lease, the California Department of Consumer Affairs recommends that you first talk to him to resolve the problem. If you deal primarily with the building's rental manager, talk to her first; if she's unwilling to help, contact the owner. The lease should include a contact number and address. If you can't get satisfaction, you can sue in small claims court. If the condition of the pool breaches the warranty of habitability, you might have legal ground to withhold rent or move out.
Just as the landlord has a responsibility for fixing problems, tenants have a responsibility for not causing them. If you or your co-tenant dump trash in the pool or otherwise damage it, or if you break the rules for pool use, the landlord may be able to cite that as grounds for eviction. On the other hand, if the landlord restricts your use of the pool out of discrimination, that's illegal. California law bans discrimination based on race, religion, gender, disability, sexual orientation or physical appearance.