Water in oil

I traded two jet skis for a 26

I traded two jet skis for a 26 ft wellcraft with a 260 horse mercruiser engine. The problem I am having is the exhaust manifold on the port side had 2 very pronounced cracks in them I replaced the manifold and ran the engine for about an hour checking the oil every few minutes and it looked clean. Now a week later I went back to get the boat ready to go in the water and found what looks like chocolate milk in the oil I fear it cracked the block over the winter but why no indication when it was running? The temp never got over 140 and it sounded very strong any suggestions would be great I really dont want to replace the engine this year with it being the end of the season does anyone have any temporary fixes to get me through another month or so. Thanks for any help. Bob

Don't know of any fast fix

Don't know of any fast fix without knowing what is leaking.
But if the water is in the oil unless it is a bad gasket between the oil & water system there is no fast fix. (head or intake gaskets)
If it's not a gasket it's either a cracked block or head.
I doubt it's the head because it would need to break in a spot that let the water in under the valve cover or the intake so it drained to the oil pan.
I've never seen this happen but that doesn't mean it can't.

You may not have run it long enough for a small crack or bad gasket to put enough water in the oil to be seen at the time.

My guess is the block is broken / has small crack under the intake.

Pour a bunch of stop leak in a test tank.(just kidding) Hope it's a easy fix whatever you find.