On the original question about Beta-Kubota, I've seen many of these engines perform very well. Never seen Beta-somethingelse
I have marinized a few Kubota myself mainly the Kubota V1505 and they are bulletproof.
I also prefer installing keel cooling and dry stack over wet exhaust but I would never install a tractor manifold in a boat. A tractor or a truck manifold is designed to lose heat via the wind generated by the radiator fan or the speed going forward in an "open box" environment.
I grew up driving open barges powered by old truck engine unconverted, from the orchard to the market loaded with fruit boxes and have seen my fair share of dark red manifold slightly glowing in the dark.
The best of both world is to still use a marine manifold to suppress half of the heat generated by the exhaust, and the dry stack to avoid rusting pipes. All the cooling systems used in marine environment are a compromise but I think the above is way better than a wet exhaust where the manifold and the riser suffer constant corrosion particularly in engines used only occasionally.
If there is no chance to fit a dry stack, the best way is to include the manifold in the coolant circuit (fresh water) and pump raw water through the heat exchange and straight out the riser and the exhaust. This way only the riser will need to be changed every 4-5 years or so and not the manifold.