1. Shut Down the Heat
The most dangerous part of a leaking hot water heater is trying to clean up scalding water. Electrical shorts can also be an issue if the water leaks into the power source. Shutting down the power at the breaker box will help you to avoid both of these issues. Often, the water heater will be on its own breaker, so it won't affect the rest of your home when you cut off power to the water heater. If you can easily access the heater without burning yourself, then you can shut down the power directly on the water heater by using an off switch or switching off the pilot light.
2. Turn Off the Water Supply
Next, shut down the water to the hot water heater to reduce the amount of water that can leak out. There should be a water shutoff valve near the heater. If you can't find the valve specifically for the heater, then you will need to turn off the water main until you can locate the heater-specific valve. You can typically find the shutoff valve by tracing the water line as it leads to the tank. The valve will be somewhere along this line.
3. Drain the Tank
If water continues to leak from the tank even after you have shut down the supply, then you need to give it a safer outlet. Near the bottom of the tank is a drain valve with a hose attachment. Attach a garden hose to this valve and then route it out a window or into a floor or bathtub drain. Open the valve so that the water can flow through the hose. Keep in mind that the water coming from the valve will be hot, so use caution while draining the tank.
4. Call a Plumber
Once the immediate issue is under control, you need to call a plumber ASAP. In many cases, a cracked tank will need to be replaced, but sometimes the damage that causes the leak is much more minor. For example, a failed valve seal or broken water line may be the cause of the leak. In this case, the plumber can simply repair the problem.
5. Address the Mess
Start soaking up the water before the plumber arrives. It will be easier for them to work in a relatively dry environment, plus prompt cleaning will prevent water damage. Use towels and wet mop to absorb the standing water, and a large fan to dry any remaining condensation. Wipe all surfaces down with a bleach solution so that mold doesn't grow.
Walking in on a leaking hot water heater is never fun, but with quick action, you can minimize the damage until a water heater repair contractor arrives to fix the problem.