They have a whole drive unit that drops the prop downward from the crank line.



The lower unit also allows the prop to be turned left and right for steering, and trimmed up and down.

Most of the difference in a marine motor relate to:
1) The cooling system, I'm not sure but I think they might not have the same drilling on the frnt edge of the head since they dont' ahve a normal water pump. In addition, the timing covers has a fitting on it so a cam gear driven water pump can be fitted.
2) The exhaust manifold - a big water cooled beast.
3) The intake manifold, and cars, are different. The cams are different too, usualy a 'P' grind. Couldn't tell you exactly what the driving characteristics are, but in general the boat motors were designed to just have the throttle slammed open and left that way for hours at a time.
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'63 PV544 rat rod, '93 Classic #1141 245 +t