Lots of boaters have dual batteries for different reasons... stereos, speakers, radar, trolling moters, bilge pumps, etc etc etc. So they use one battery just for the engine, and the other battery for when the engine isn't running. That way their engine battery is always fresh and not run down.
And if, for some reason, they need the extra juice from the second battery, it's available with just the turn of a switch to the "both" setting.
Additionally, when set to the "off" position, it prevents any unwanted drain on the batteries by isolating them from the electrical system. (Useful if your boat spends long hours on a trailer between trips to the water).
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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