CHARGING A 2ND 12V BOAT BATTERY

I want to use and charge a 2nd utility battery on my boat, but I don't want use of the utility battery to deplete the starting battery. How, 12 V Diode(s) in series?

There is a device called a "split charge relay" that does exactly what you want. Split charge relays are used on boats and campervans to isolate the 2nd battery so that it can be used while the starting battery stays charged. If you search for "split charge relay" you will find lots of alternatives.