I've never seen a renter's policy that didn't have some property liability coverage.
They have insurance for the unit damages that covers their unit. You have to pay 15 dollars per month for that, but it is waived if you get a policy that covers your personal items and has them as an interested party, but it says nothing about covering the unit itself and most I looked at are only for personal property.
The HO-4 Renters Policy also known as Contents - Broad Form has two parts. Part 1 is Property. Part 2 is Liability.
The Property part covers the insured's personal property. Ergo, no building/unit/structure coverage.
The Liability part covers the insured for injury to people or animals and damage to the property of others due to his negligence. Anyone making a claim against the insured has the burden of proving that the insured was negligent. Of course, if he's not negligent (at fault) the insurance wouldn't pay
As I mentioned earlier there is no liability coverage for damage to the dwelling unit rented to the insured unless his negligence causes a fire, smoke or explosion.
If his negligence caused the water damage to the unit, his policy has no coverage. He would have to pay for it himself. If he was not negligent, he would not be liable for payment, the unit owner's policy would have to pay for it because the HO-6 Condominium Unitowner's Policy Property section covers the interior parts of the unit structure that are not defined by the CC&Rs as "common elements."
Any other questions involving insurance, feel free to ask.