The item you linked is an oil drain valve. Your water heater sounds like a Suburban and if so, it requires an Anode Rod. Anode Rods are linked above.
Because it works with electrolysis, its threads require contact with the tank. You can still use Teflon Tape on the threads. They'll cut it in enough places to make that contact while still keeping the thing from leaking and allowing you to
remove it easily. You'll find crusty deposits, so point a fine water jet into the hole to drive them out. There's a "wand" that helps.
When you install that anode, wrap black electrical tape around the hex of the plug part so the socket forces onto it. Then align with your socket extension so it's all one straight thing. Makes it easy to start straight.