This is the simplest battery layout of all.  There is just a single battery and an on-off switch.  The switch is an ABYC requirement and is there so that the circuits can be turned off in the event of a fire or other malfunction and to prevent the battery from discharging when the vessel is not in use.

 

Simplicity is sometimes best but this arrangement does have the disadvantage of having the engine become difficult to start if the battery becomes discharged from the other circuits that are using it.

 

I havent shown circuit protection here but the wire to the breaker panel is required to have circuit protection immediately after the switch.  I prefer to use a circuit breaker rather than a fuse for this because if the main fuse blows you can be left in the dark hunting around for a replacement.  I have omitted the negative wires for clarity.

 

See below for a variation on this

 

This slight variation on the simple layout uses a Victron "Battery Protect" in the circuit.  This device is designed to shut off the circuit when the voltage drops below a certain level.  The shutoff voltage is adjustable and it would take a bit of setting up, but this would be a valid way for small boats to have just a single battery without being able to discharge the battery so much that it wouldnt be able to start the engine.

 

Like everything, the devil is in the details.  Would you have the bilge pump and VHF bypass the battery protect?  If you didnt you might find some unfortunate consequences in the event of an emergency.