1982 supra-celica engine problems. plz help

[bjm007]

Hi everyone! Im a young teen who was given an 1982 supra celica in almost perfect condition.. It only had 45,000 miles on it and was garaged its entire life... All of a sudden the car started running really rough, i opened the hood to check if a wire had come off and i noticed water in the third plug hole back from the front of the engine. Dried it all of and took it to a local toyota mechanic. He did a tune up and a compression check. He said the compression was good and after the tune up the car ran smooth once again. but the next day water was back in the spark plug hole??(and it even blew the spar plug wire right off the plug). also after getting the car back from the mechanic theres a knock at after starting the engine and at low rpm...This knock goes away with an increase of Rpm... any ideas about the water in that plug hole and the knock would be greatly appreciated.. im just lost here and dont know what to do?