as Stu says, a wiring diagram for your boat... then a good book explaining how circuits work (charlie wing)... then read and study the diagram/prints for your boat. eventually it will begin to make sense.... anything you read or hear about the subject from someone else, other than the basic concept of 12v electrical, may only confuse you.
with the diagram, you can learn the basics at the same time as how it applies to your particular project.... way less confusing than hearing how others have it all set up on their boats and trying to make sense of how that applies to what you have
things that are
usually common in a basic system is....
the grounds all go directly to the same buss bar (-negative-)... the buss bar has 1 large large wire going to the battery
the hot wires to the devices come from individual switches, and fused either before the switch (preferable), but sometimes after the switch.
the switches usually all get their primary power from the same buss bar (+positive+). and it has a large
fused wire coming from the battery.
the buss bars prevent a mass confusion of many wires being connected directly at the battery. some boats are not set up with buss bars/panel but its much preferred that they are.
there are some exceptions, but not very common in a simple basic circuit design...
