When you have a puddle of water under the water heater and there is a floor drain below the water heater, you have to start looking at the water heater first to see if there isn't a hole. If the water is running constantly, then it's probably the water heater leaking. If the water comes up when you use other fixtures in your home, it's probably a blockage where the floor drain and the branch line come together. You should call for a drain technician to clean the line.
If the water heater is leaking, it's time for a new water heater. A qualified plumber can install a new unit.