The rental agreement usually controls in these situations. It should spell out who is responsible to maintain the AC, furnace, hot water heater, etc.
Why did the hot water heater leak? If it's old and rusty, then it is probably the landlord's responsibility because it is a fixture. You come and go; but the fixtures are part of the building.
These situations are usually best handled by the party with insurance... again it's probably the landlord.
As for the mold, you need to argue that the carpets and padding damage are a direct consequence of the bad hot water heater and you may need to argue that the mold makes the place uninhabitable for you, if the mold gets bad. That would be a constructive eviction. They can't force you to stay in a dangerous area when they (the landlord) caused the problem by failing to keep the premises maintained in a safe and habitable condition.
This is general information only and this does not create an attorney-client relationship. I am a Florida personal injury and wrongful death attorney. For information in my fields, visit my website and, while you're there, click on the ad banner for travel, gas, or food vouchers. They change periodically; so keep visiting www.inawreck.com.