Where is it leaking from? A valve, through the walls of the water heater, out the bottom? It could be as simple as replacing the temperature/pressure safety valve. They sometimes go and the actual heater is fine. If it is not a valve, then the heater probably needs to be replaced. If you don't have a drain, or if the water will cause damage to the surrounding areas, you may want to turn off the gas to it and shut off the water coming in (hopefully, you have a shut-off valve for the water heater input). You could also either put a hose on the drain valve or use a bucket to empty the thing. Wait until it cools off first, though - could take many hours depending on how hot it is set to and how good the insulation is. The thing won't blow up (unless the safety valve is defective), but it could dump all of its hot water on the floor and if you haven't closed the input valve, the cold water will continue to flow, creating a mess. The leaking water could douse the pilot light, but again, the safety circuit should shut off the gas once the thermocouple cools off (pretty quick if it got doused with water).