I have a 22' Chaparral with a Merc 350 300HP. I do a lot of lake
boating, long days, and would like to have a reserve battery to start
the boat in case the primary drains while docked or just for
emergency.

I'd like to wire in a combiner and switch as show here
http://www.skingcompany.com/manuals/C50.pdf

I am not sure what amp rating I need for the combiner and switch. Can
someone help with this. I don't care about saving pennies, want the
best system for safety.

Thanks,
Bob