Boat Auxiliary battery - this simple ok?
It is common practice on "simple" boats to have two batteries with a "1-both-2" switch to select one or both batteries as needed.  It is usually suggested that you put the switch on "both" while the engine is running, to charge both batteries, then switch to "1" or "2" when the engine is off, so only one battery is discharged - usually, you should alternate which battery is used with the engine off to equalize wear on the batteries.  I haven't heard of any surge problems as suggested by WhatRoughBeast when using this system.

A more complex system would have dedicated "start" and "house" batteries, with the start battery only used for engine related things, and the house battery powering all non-engine loads (lights, electronics, fridge, etc.)  There are various battery combiner or isolater devices to ensure that both batteries are charged when the engine is running, without user intervention.  (On my boat, I have two alternators, one to charge the start battery, and the other for the house battery.)