I pulled the boat (1987 Supra Saltare) out this last weekend for the first time this spring. I hooked up the fake a lake and started it right up, I only ran it for a minute or two. The engine was rebuilt less than 100 hours ago, and runs like a top. I cleaned it up and felt like it was ready for the maiden voyage of the year. We headed down to the lake and launched the boat. It almost immediately started to overheat, I pulled it over to the dock after running it for only a minute or two and noticed it was very hot. I shut off the engine and suspected that perhaps the impeller needed to be replaced. It has been two years since it was replaced. Then I pulled the dipstick and it looked milky. I pulled it back on the trailer and took it home. After letting it sit a day I pulled the dipstick again and it's definitely water in the oil. I am now nervous that the block somehow cracked, or it needs a new head gasket. I am pretty depressed about it, I have put a ton of time into this boat over the last few years.
Is there anything else it could be? Anything I can do to diagnose it?