Depth gauge functioning?

[JASinIL2006]

Hi I have a '97 Larson SEi 186 I/O bowrider that is new to me. I recently took it out on a big river for the first time and the depth gauge operated somewhat erratically. At times, it seemed to be working fine, showing a depth that seemed to be about what I'd expect. At other times, though, the display would show no numbers... even when the display was blank, though, I would get a warning beep when in very shallow water (i.e., approaching a sand bar).

The display going blank seemed to happen irrespective of whether I was idling, on plane, or in between.

This boat is new to me, and it's my first boat, so I don't know if there is some setting I need to make, or if the gauge is flaky or what. Any ideas? Any pointers to directions for settings for these gauges?

Thanks!