I am having a problem with my boat engine. GM 350, with mechanical fuel pump. The crankcase oil is being contaminated with something. I doubt if it is water, because the oil simply gets thinner, not milky. I am suspecting the fuel pump is leaking into the timing chain area.
Any thoughts?

Inboard engine fuel pumps and carbs are designed to drain into engine if they leak/fail. This is a safety feature designed to keep your boat from blowing up. NEVER replace anything with regular automotive parts.

With that said, it could very well be a carb or fuel pump problem.

Poor compression or bad spark on one or more cylinders could also cause contamination.

Do you have trouble starting this engine due to flooding? Flooding immediately contaminates the engine oil. If you flood the engine badly the oil should be changed right away.