This image shows a pushrod Falcon six motor which uses a large plenum chamber and bolted on alloy runners that benefit mid-range torque.Long runners can give more than 100 percent volumetric efficiency which can be free power increases up to 20 percent and long runners can give performance motors enhancements such as better cylinder charging at low RPM and idle speed can be reduced to a slow tick-over,ideal for a boat.A carburettor (and EFI systems) can be installed in a safe place sealed from inside of the boat.A combination of short and long runners with a switch over would give benefits in a greater RPM range.