I live along Mobile Bay in Alabama, so there is no concern with freezing pipes this far south. (It is remarkable in this region that residential irrigation systems are routinely buried just below the turf surface.) When the engine is shut down, most of the water has already been blown out the tailpipe, leaving very little water reservoir to freeze near the check value. Besides, my check value is not quite at the lowest point between brass exhaust and the tailpipe, the tailpipe exits the hull a few inches above the water surface, and my slip is in a very well protected area with virtually no swells that would force much seawater back into the tailpipe.
Nevertheless, it is feasible that some hardcore soul sailing in subfreezing temperatures could encounter a frozen check value, causing the exhaust fumes and seawater to backup into the engine manifold.
The riser actually extends about 8 inches above the manifold port when you consider the tee and the upper elbow. In this instance there is minimal space to extend the riser much higher before the exhaust system would be too close to the cockpit floor for my tastes. To the best of my knowledge this meets the original riser specs.
As for the material found at yaughtsurvey.com, there is always more than one way to skin a cat. Personally, titanium is beyond what I am willing to spend, cast iron has already proven itself to be of inadequate corrosion resistance, while brass yielded a reasonable price point and expected to be of low maintenance. My wife would insist on that 4 karat diamond before I upgrade to a titanium exhaust system.
My boat is sort of in fresh water…in a slip fed by a fresh water “creek”…gets more saline contact once I get into Mobile Bay and The Gulf. I have more trouble with slime on the bottom. The underwater parts remain virtually free of any barnacles. I have not converted all anodes from zinc to aluminum alloy yet but am heading in this direction only because of the low saline environment and the increased galvanic resistance of aluminum compared to zinc. (Aluminum alloy anodes are more corrosive than the brass exhaust components.) I consider this conversion an experiment until I am sure a corrosive “skin” that eventually covers aluminum does not negate its sacrificial intent like painting over anodes.