water heater plug

If your heater is a Suburban, the "bigger hammer" will probably work. If it is an Attwood and someone put a steel plug in the aluminum tank, you could well be looking for a new heater.

operate the unit to temperature, then try cracking the plug before it comes down to ambient temperature. dont do anything more than crack the initial freezing that is keeping the plug in, requiring you to use abnormal initial torque to break free, then let cool and shut off pump, and drain pressure from heater, and then remove plug...good luck

And/or tap on it with a hammer but the hammer drill should do it.

One trick that I have used successfully is to TIGHTEN the plug first THEN back it out. Sometimes it takes an impact to break it loose. In other words "get that bigger hammer" and go the wrong way first :)

One good way to prevent them rusting in is to make sure the plug is left IN when in storage. Some folks leave the plug out thinking the tank will dry (which it doesn't need to do and the result is the threads rust.

If you have an Atwood water heater they are supposed to be the nylon plug that will not corrode. They are cheap to replace but some people go and buy a metal one. They will always corrode in place.

If it is rusted, it would be a Suburban water heater which is a steel glass lined tank. Get proper sized six point socket, six inch extension, breaker bar ans a cheater about three feet long.
This is an anode for the tank. It will a a composite probe inside the tank.
You may need need anode. They should be checked at least every year or to be safer every six months depending on water.