Tom,
I think that a fuse of only 20 amps might be too small for your key switch. The starter solenoid can draw more than 20 amps inrush current. You might want to protect the circuits after the key switch rather then before. I know of one boat that had the alternator wired incorrectly (12 volts was supplied to the tach pickup) This resulted in a full fielding all the time and boiled out 8 batteries and melted the wiring harness from the engine to the panel. The worse part of this was the wiring harness ran right on top of the metal fuel tank!