Fuel tank siphoned from water tank and into engine

[WoodFamSkipper]

Here's a topic that I ran into the other day

Viewing a boat for sale that had multiple tanks, for different purposes.
A prev owner had decided to convert a Fuel tank into a greywater - but didn't terminate the feed into the fuel tank
The new owner, was running engine until it came to a grinding halt.
Greywater had gone to fuel tank and up and into the engine - more than a Racor bowl could skim and filter

What would you make of an engine that had been through that kind of process??
Does it take a rebuild to fix? Can it just be drained and not suffer too much other than it's initial halt

Boat is now for sale, with 800 hours on the block - but I'm wondering if it could have any longer lasting effects of drinking up a bunch of liquid kryptonite?

I remember my Dad's diesel van in getting flooded with water in the air intake, back in the 90's, and that equaled a dead engine and written off vehicle