1991 3.0 mercruiser water in oil.

I just purchased first boat this year a 1991 searay 170. When I got it I took it to a boat mechanic to go through and they told me it needed a new carburetor because there was a lot of gas in the oil. They changed oil, put a new carburetor on and new spark plugs. I took the boat on the water ran great however it was taking on water looked like from the back of boat. I took it back in and it needed new gimbals, bellows, water pump and few other things. I got it back out on the water and again ran like a champ. I check for any leaks and there was no leaks while it was running however it leaked if it was just sitting there and I could not find where the leak was coming from. I saw no dripping from engine. Also after about 30 minutes or longer on the water the engine seemed like it was getting bogged down so I just turned it off and when I turned it back on it ran fine again. I checked the oil and the oil was milky grey. So I changed the oil and ran it again and after the run I checked the oil and milky gray again. I’ve seen a lot of post on here where it could be cracked head, head gasket or exaust. I’m wanting to do the work myself and believe I can just don’t know where to start. I did pull one plug out and very hard to loosen to get out and seemed wet on the end. This is a 91 3.0 mercruiser in-line 4 cylinder.

Ouch! Water can get in from all those things you stated plus a bad exhaust system. You need to check them all out. Start with a compression check--that might show you if a head gasket is blown.

Jeff

Did a compression check and all cylinders were 160.