Water heaters sometimes leak from the drain valve or relief valve. Those valves are easy to replace. But if a leak is coming from the tank, you've got serious trouble. The tank is lined with a thin coat of glass. Eventually, that glass begins to crack, the steel begins to rust away and a puddle appears. Left alone, the tank will rupture, causing an instant flood. It may take months for a leak to become a flood, or it may take days. But it will happen. Don't gamble. Replace that leaking time bomb now.