Having experienced my D3-110 failure firsthand, my theory was that the water infiltration into the cylinders was a gradual thing and was likely the result of excessive back pressure through the exhaust. I cannot believe that the flapper would have let a lot of water into the muffler while sailing or motoring, given the high loop above the muffler. If the engine were "getting weak" so to speak or there were any sort of water buildup at the muffler, I have to assume that my failure was the result of water sloshing back up into the engine upstream of the muffler. There is a valve at the bottom of the muffler, and I now periodically drain it just to be sure.
My theory about too much back pressure is also the reason I increased the size of the hose between the muffler and the flap, and also why I significantly shortened the hose vs. the original long loop.
Certainly, Amel's recommendation to run the engine routinely is a good one regardless of the configuration.
I am now a happy camper with my new D3-150.