I'll pretty much echo what everyone is saying about a separate fuse for each toy. As jyaski says, the engine stuff wil lbe fed direct form the engine harness. Google Image "Merc wiring diagram" and have a dig around what you find - that will take care of the engine related stuff.
I have also put a switch per toy as well, simply in case I have to isolate one for some reason (fault finding at sea kind of thing ) . There is an argument for a separate gauge illumination switch as you may find on a partuicualrly black night your night vision is trashed by the glowing tacho under your nose.....
Also check your main battery cables - I found after a rather reluctant start last month that I had a nice console heater in the form of the jump wire between the batt +ve and the main isolator. Chopepd it and it was very black inside..... 35 sqmm welding cable makes remarkably good replacement.
Only other high level suggestion I would make is to take a length of wood, and park it across your toobs. Try if at all possible to fit everything above this level. Simply because if you ever get swamped, the whole lot will die of corrosion in a matter of hours or days. Not of course that I speak from experience on this matter.....