Mercury - Water Level Gauge Gray Face 2-1/8" (52 mm) 79-8M0065972. Overview. The Mercury Water Level Gauge (79-8M0065972) lets you monitor your onboard freshwater or livewell tank levels with precision and ease. It features a gray face, white numerals, and a glass lens for a sharp, easy-to-read display that fits right in with Mercury's OEM gray-face gauge line. The gauge installs in a standard 2-1/8" (52 mm) dash hole and works with SmartCraft-capable systems when paired with compatible tank sen