The fact that the oil and water are not mixed is a big clue, pretty much eliminates the raw water system as a suspect. (Unless, as mentioned above, the motor lives below the waterline)
For my $.02 worth, How about a bilge pump failure or a delayed startup of same?
The ER could have flooded, with a sticky float switch kicking in late.
The water could have then entered through the dipstick port.
You can lead a horse to water,
But you can't make him ski...