Depending on how long the previous owner had been running the tank without an anode rod the inside of that tank could be in rough shape.....
When you take that plastic plug out keep an eye on how many metal fragements come out with the water. The annodes job is to protect the inside of the hot water tank from corrodeing, without one the inside of the tank begins to corrode, eventually to the point that the wall of the tank will give way.
If not too much metal debris comes out buy a new anode rod, and after applying teflon tape, see if the it will thread into the tank, or if the threads being damaged was the reason why the previous owner went to a boat plug.
Sorry this could end on needing a new tank if it has been without an anode for long enough.