The purpose of a battery switch on a boat is to:

The primary purpose of a battery switch on a boat is to isolate the battery. Isolating the battery is essential for several reasons, including safety and maintenance. When a battery is isolated, it disconnects the electrical system from the battery, which prevents accidental discharges and helps avoid issues such as electrical fires or damage to electronic devices. Additionally, isolating the battery allows for safe maintenance and servicing without the risk of short circuits or electric shock.

While controlling charging and preventing deep discharge are important considerations in the overall management of a boat's electrical system, they are not the primary functions of the battery switch itself. The switch's main feature is to provide a way to disengage the battery from the boat's electrical systems completely. Improving fuel efficiency does not directly relate to the function of a battery switch; rather, it pertains to other aspects of the vessel's operation. Therefore, the correct answer underscores the crucial role of the battery switch in isolating the battery for safety and operational integrity.