What would make water suddenly mix with the motor oil ?

The boat has been great and only has 900 hours on it in 30 years.

The boat would not start after the hurricane left my area. The motor
would not start after many attempts. I assumed it had just been soaked
from the 12 inches of rain.
I started checking everything from fuel air spark to oil. when I pulled
the stick it was milky gray.

I drained it asap and now have fresh oil in it. I have not attempted to
restart it until i get advise.

The engine appears ok to the naked eye. All the gaskets, elbows are ok
when looked at. My neighbor has the same boat/engine. he had water
enter through the carburetor spark arrestor.

The water would only come from a cracked intake/water cooling manifold
or an exhuast elbow right ????

Again, motor has been healthy and was tuned, oil change in June 2006.I
went for a three hour ride just before the bad weather arrived.

Thanks in advance.