I agree with NYBo that each circuit should be individually fused. Although in your schematic one 20 amp fuse might be adequate protection to feed all the loads you’ve shown, it’s important to quickly isolate a problem causing a blown fuse on a boat. A single fuse for everything will leave you puzzling for quite a while if it ever blows.
The location of fuses is usually as close as practicable to the source where the wire is connected. The distance also depends somewhat upon the degree of hazard to the wiring such as how it is routed and protected from damage. For example, ABYC standards allow a greater distance if the wire is contained in a raceway.
The battery switch you have pictured is capable of switching between batteries as you have planned for a future second battery, so the wire from the battery to the switch may eventually be larger than #12. If this wire is intended to carry starting current for your engine, a larger cable than #12 is required and a fuse is NOT required between the battery and the switch, assuming the switch is installed as near as practicable to the battery. If you are going to feed the heavy switch from the battery with only #12 wire for now, I would put a main fuse of 20 amps near the battery.
Also, I would leave the switch that is fed common to the outlet as a spare, rather than using it to feed critical equipment such as the bilge pump. You don’t want to lose operation of the pump because of a faulty air mattress pump or whatever that was plugged into the outlet last weekend. - Grandad
