Don't feel bad. No one is born with knowledge. It is gained.
Click the link to a sample picture of a in-line fuse.
One line to battery positive, and other to the device. The fuse must be rated by the manufacturer to provide proper protection.
Unless you make a mistake of using different batteries with higher voltage rating, you really don't need fuse protection. Or if you charge the battery with device connected, thus the charger puts out higher voltage.
In saltwater environment, the less "exposed" connection points the better.
All of the connections should be sealed with shrink tube or plastic paste.
A dab of electrical grease at the monitor connection is recommended too.
https://www.delcity.net/store/AGC-He...20group%20%231
I hope above helps.