Found Very Concerning Issue

[billybronco1]

So this 36 I just bought has a rebuilt engine with about 3K miles on it, told it has aluminum heads. Today I started it up and noticed when it was cool lots of black water or moisture coming out of the exhaust, once it warmed up it seems to clear up and go away. I called the previous owner, and asked him about, he said it always did that for the last 7 years and never had to add coolant. I decided to pull the spark plug to get the number off it and what do I see? Anti freeze sitting on the valves. Seems its the middle two cylinders on the drivers side only.

What do you think I should do?