Water can get in the oil a lot of ways...is it a salt water cooled motor, original? If so....
intake manifold can rot out right under the thermostat housing and put water right in the cam valley right in the oil...
cyl heads/block can eventually rot out more so the heads, but this is relatively rare..the cooling passages in the heads where they match up with the head gasket can erode from salt water use and eventually cause the HG to not seal, and that can put water in a cyl or the oil
a bad gasket between the exhaust manifold and elbow can allow water in a cyl, or a rotted thru elbow or manifold same thing.
In salt water we usually say the manifolds are elbows are good for 5-7 seasons, inspect sooner if you see rust stains coming from the joint between the manifold and elbow....
I learned about the cooling passages issue when I was doing a top end overhaul on my '88 4.3 V6, it had blown both head gaskets about 3 years after a bad overheat. I took the heads to a local machine shop (SK Speed in Lindenhurst LI) and they found that both heads had cracks in the center cyls exhaust ports, but the cooling ports were also eroded, and the machinist felt they would not seal after decking them. So I installed a set of re-man marine V6 cyl heads + new Fel/Pro gaskets.
pic of new heads and top end overhaul:
https://www.dropbox.com/s/8fhnjbtutfjejkm/4.3 reman cyl head 2.JPG?dl=0
https://www.dropbox.com/s/1yyytu26xgohers/IMG_2211.JPG?dl=0
https://www.dropbox.com/s/55j6h4dd1u8fv0k/IMG_2273.MOV?dl=0
