A few more pieces of information will be helpful: (I don't think there was ever an electrical schematic published)
Did the boat just stop running one day?, or did you buy it this way??
Do you have a good battery?
When you turn the ignition on do you have power to other electrical items, (radio etc)
I see a tool box back there so I'm wondering if you can check if you have power to the starter motor?
It could be a few things but these are pretty simple electrical systems. Follow the current, where you have it, and where you don't.
That is a pretty green.
Best of luck
David