There is a standard marinized OMC 2300E series 350 Holley carb which uses the return line and hooked air horn required by most US Coastguard authorities. It's a simple and effective carb, and it allows good torque and power with the later model heads if its got a direct mount 2-bbl conversion. I'd use that carb as opposed to a 5200 Holley Weber in a boat, as the staged secondary is very sensitive, and it would be constantly moving on an off secondary at about 2500 to 3000 rpm in a boat.
I've seen a brillant 1.62:1 prop system used on Mercury Marine 305 and 350 engines I've gone through in my job as a Biosecurity cleaner. We've devanned a number of ex California boats, and Kiwis prefer the reduction drive inboard V8's in boats which a way too heavy for these puny engines. The reduction drives should work a lot better in smaller I6 boats like yours.
The 200 is best linked with a 264 degree cam, a 350 2-bbl, and post 1975 head to give enough power at 3500 to 4500 rpm, and enough torque from 1500 to 2500 rpm. Cold cranking compression should be about 175 psi, and to do that, you'll have to retard or advance the cam via changing the valve closing event from the stock grind. Stock compression is very low, but you can bump it up a point to 8.8:1, and reduce peak advance on a Duraspark ignition system. Heat build-up on a boat is best removed by a well designed heat exhanger, and a well cleaned deslagged block and head which hasn't been bored and shaved within an inch of its life will be detonation resistant head. The heat to the carb can be isolated by a phenolic spacer in top of the direct mount log head conversion.
Down here in New Zealand, the Hamilton 2-stage jet units are used often, but we find a stock log head 200 cube engine is too small to make enough mid range power at 3500 rpm for a four seater boat. When you go to freer better breathing X-flow 3.3, it tends be worse lugging at lower rpms like 3500 rpm. It tried to buzz up to 3800 or more, but then the jet unit doesn't suit the 3.3 nearly as well as the bigger 250/4.1 engines...they are much easier to cruise at 3500 in adverse conditions. The way to get 250 style power from a US 200 is to bump up compression, and improve cam profile and breathing. Due tothe rod ratio, the small intake and exhast ports, the US 200 engine is a lot better coping with bigger cam profiles than the Aussie 200 cube engines we use. Ours use long 6.3" rods and big 1.35" ports, and when you use bigger cams, they raise the rev range too high, and you loose the ability to lug at low rpms.
