It does not sound like there is anything wrong at all. Were you there for the winterising process?
When an engine is winterised, antifreeze is run through the cooling system. The antifreeze is mixed with water - 50/50, 60/40, maybe even 70/30. The antifreeze that a lot of people use for diesels (and never for water systems) is GREEN. When it's mixed with water it is WATERY GREEN.
When the cooling lines are full of antifreeze, the impeller and the thermostat are removed for the winter and their covers are replaced. During this process, some of the WATERY GREEN solution falls into the pan under the engine.
When the engine's cooling system has been filled with antifreeze, a lot of people will the pour the remaining anti-freeze into the bilge, and pour some more into the engine pan.
This could be the watery green mixture used in the engine, or it could be a dilute of pink or orange antifreeze as well. The pink stuff is okay to use in potable water systems, the orange and green stuff (ethylene glycol based) are not.
I think your engine is fine and that your boat was winterised properly. Do the same thing next year.
Either you have to go down to the boat every 24 hours and keep the bilge BONE dry or you have to put a goodly layer of antifreeze into the bilge for the winter.
The one thing that you don't want to do is allow two or three inches of water to lie there and freeze and thaw and freeze and thaw the whole winter long.
It damages the boat.
Good Luck !