From your description (plastic plug etc.) I will assume your have an Atwood Brand water heater.
The tank is made of aluminum alloy and does NOT use an anode rod of any type (as suggested). The drain plug is nylon for a reason....so that it will get damaged vs the tank drain hole threads.
As suggested.......chase the threads with a 1/2" NPT tap. You may need to temporarily remove burner tube (which is easy to do and could probably use a cleaning anyway).
When chasing the threads with tap visually inspect area around threads for any cracks.
After chasing the threads a new nylon plug should screw in easily.....finger tight. Then tighten 1/2 half turn with socket.
Should seal...no leaks