A leaking water heater
A water heater that is weeping at a fitting and a water heater whose tank has given way are two different events. The first can wait for a scheduled call. The second empties forty to eighty gallons onto the floor and keeps refilling.
First: stop the supply and the heat
Close the cold water shut-off on the line entering the top of the tank. If it will not turn, close the main. On an electric unit, shut the breaker off. On a gas unit, turn the gas control to the off position — do not simply turn the thermostat down.
Heating an empty or partly empty tank damages it further and, on an electric unit, can burn out the elements within minutes.
Where the water is coming from
Water at a fitting or a union is usually a connection problem. Water from the temperature and pressure relief valve, or from the discharge tube running to the floor, means the tank is relieving pressure — a symptom of something else. Water coming from the bottom of the jacket with no identifiable fitting above it generally means the tank itself has corroded through, and a tank that has failed does not get repaired.
What it does to a finished basement
Most water heaters in this part of Maryland sit in a basement, frequently in a utility area adjacent to finished space. A tank failure sends clean water across a floor at volume, and it moves under baseboard and into carpet padding faster than it can be mopped. The water itself is not contaminated, but the assemblies it soaks — padding, subfloor, the bottom plate of a stud wall — hold moisture long after the surface looks dry.