|I am no expert. But I think this is fuel, and I think it is perfectly normal.
When the engine is running hot, you have hot exhaust pushing hot cooling water through a common exhaust/water hose pipe and through a water lift muffler and out the stern. Hot exhaust, hot water - mix pretty well.
Stop the engine and everything gets cold. Some water and fuel is left in the water lift muffler, and some in the hose before the lift muffler. The exhaust gas, which will includes some unburned fuel, as these engines are not particularly clean burning, separate, as water and gas or diesel mixed will do.
So, when you start up the first thing to exit is the cold water and separated fuel, along with various chucks of carbon in the exhaust system. So you get a small film of fuel and carbon on the water until the gases and water are hot enough to mix again and make it look clean.
Anyway, that is my theory. But I'm no expert.
My 1GM10 has been doing this ever since I got it, and I wouldn't give it a second though. Just annoying that it fouls the rudder and transom a bit as it warms up.
Barry, Oasis, 1987 Mk V