Here is a generic boat instrument panel wiring diagram. What you need to understand is that marine gauges require sending units that are compatible with the gauges. The oil pressure sender from a VW engine will not work. You need an "international guage" oil pressure and fuel sending units. The tachometer must be compatible with a battery ignition system which is a typical I/O or inboard tachometer. Marine speedometers are not cable driven. They are "pitot" (water pressure driven) via a small plastic tube from the pitot at the stern to the speedometer head. If you are using a cable driven automotive speedometer I have no idea how you would make that work. If there was a way to connect the speedometer cable to your drive system, chances are the cable drive ratio would be incorrect.
SmartCraft is a proprietary vessel digital data network standard. SmartCraft gauges are designed to be connected to a proprietary data network. A network node in the engine sends digital data about the engine parameters on the network. Each gauge decodes and displays its particular parameter.
Trying to use SmartCraft gauges without the proprietary network and the proprietary engine sending unit will be impossible. They gauges only work with a Mercury outboard or sterndrive.