Replace the pressure relief valve if it leaks and the water heater isn't overheating.
If the water is overheating, the dripping could be because the overheating tripped the valve. Shut off the circuit breakers and check the thermostats and heating elements. A failed thermostat can cause the element to heat constantly, resulting in overheating. A heating element shorted to the metal tank or cabinet will also heat constantly and cause overheating. Watch this video to find out how to troubleshoot an overheating electric water heater. Replace any failed thermostats and shorted heating elements.
Replace the thermostat in a gas water heater if it's causing overheating.