I read something recently that stated that if you cold crank your inboard diesel for too long you can flood the engine with seawater. As the story went: in the absence of back pressure from the engine the raw water that is pumped into the mixing elbow will backflow into the engine. Is this true? If so it seems like a serious design flaw.
It is true! You will flood your engine and I agree, it's not the best design. I had to install a complete water-lift muffler and exhaust on my boat. But there is no avoiding the situation with most wet exhaust installations on sailboats. The engine is usually at or even below the waterline so that gravity cannot drain the system.
On the good side, most raw water pumps draw very little water during the slow rpm's when the starter is engaged. But they do! And the output will vary so you should test to find out.
So it's a matter of volume: How much water can your water lift muffler and hoses hold, beneath the exhaust riser and/ or point of entry once the system is full.
I promised myself I would check to see how much raw water is pulled into the water lift muffler, during engine cranking. Sorry, I haven't done it yet because my engine is a quick start.
It would be a simple matter of draining the water lift, replace plug, crank for say 20 seconds. Empty water
lift into a container. If you do it, let us know the result.