I've never seen how these engines are set up in boat but I'm going to assume they have coolant in the engine block, head, and exhaust manifold(s) and a raw water heat exchanger fed by a raw water pump of some sort.
Was there some way fresh water-- rain, boat's supply?---could fill up the exhaust manifold(s) thus get into the cylinders and seep down into the sump? Broken fresh water hose or sink drain hose or something?
Or is the surface water in your marina fresh from runoff and Keith's scenario of water siphoning in through the exhaust is the answer?
Or did fresh water somehow get down the oil dipstick tube? Or into the crankcase breather?
Quite the mystery.