My Engine Made Chocolate Milk

So I pulled the boat out yesterday to get her all cleaned up and make sure everything is working properly and when I went to check the engine oil to my surprise someone had drained all my oil and replaced it with Chocolate Milk. After sampling the so called Chocolate Milk and almost puking I came to the conclusion that it wasn't Chocolate Milk at all and some how water had managed to get into my engine oil. I'm not sure how the water got in there because after every big rain I always go check to see if any water leaked through the cover and bilge it out as necessary so I know there hasn't been any standing water in the engine compartment is it possible that over the past 5 months of sitting through all the rain we've had that moisture was able to build up inside my engine ?