Three models of fuel gauge for your battery. These monitors measure volt-amps in and out. Shows percentage remaining. Alarms can be set. Also makes an AC monitor for shorepower and genset monitoring.
A variety of electronic displays for autopilot, GPS, and other performance and navigation data. Sensors include battery state, wind speed, water depth, heel angle and others. You can read the User's Guides on line.
A couple who have completed at 10,000-mile tour of the Pacific on a 31-foot cutter talk about what equipment they chose and how it worked, including their battery capacity and management.
Multi-function monitoring system for fuel, water, sewage, AC power, DC power and other on-board resources.
Unmoderated forum on marine electrical and electronic equipment. Good advice and resources here, but you have to dig and you have to be selective.