The battery is always wired up to feed power to the starter if the "start" function is energized. To energize the start function you need a high current switch that can be controlled by a low current, (usually ignition key controlled) remote switch....when rotated to the START position......Enter the starting solenoid.
It has 2 circuits: One low current activated by the start switch in the remote control or on the dash of fancier boats. The low current circuit energizes an electric solenoid that has a set of copper high current contacts attached to its moveable shaft....movable when the solenoid is energized by the key switch.
When powered to do so, the high current contacts inside the solenoid close and direct high current to the starter to start the engine through usually 3/8" diameter screws with nuts to hold the cables attached. Battery (+) is connected to one contact and the starter cable the other.
When one energizes the solenoid, you should hear a click developing internally. If you don't hear the click the solenoid hasn't energized and the culprit could be the switch, wiring not letting 12v to get to the solenoid, the ground wire on the solenoid isn't grounded, or could be coil inside the solenoid is defective.
If you hear the click and the starter doesn't start spinning, it could be several things. Quickest way to eliminate the solenoid as the culprit is to put on some gloves and some eye protection, grasp a set of common pliers and expecting a spark, take the pliers and short across the 3/8" bolts nuts.....connect to the nuts, not the bolt threads so that any sparking won't damage them. If the solenoid is the culprit, the starter will spin. If not nothing will happen.
Next possible culprit is the battery. Have it load checked at places like Auto Zone auto parts who carry a battery tester, and ensure that it's fully charged. You need 10 volts at least across the starter itself to get a good spin up and during that process some voltage is dropped across the batteries wiring and some more internal to the battery. Side note: The reason for the load test is to ensure that the internal voltage drop, during high current delivery, isn't too high.
Next possible culprit is the wiring to-from the battery to the engine: The black, (-) lead goes to the engine block usually on a thick cable. Remove the cable at both ends and shine up the internal connections (cable terminals and places where they are installed and reinstall it good and tight......seldom is the cable itself the problem. Then do likewise the Red (+) identified cable.
Last would be the starter itself which is usually not the problem but it has to be connected to the solenoid properly and the case has to be properly attached to the engine block where the (-) battery lead is attached. Do the cleanup of these connections like you did for the battery cables.
If still no spin, then suspect the starter itself!
Good Luck.
