Depth Sounder help needed: Standard Horizon D41BD

[guy48065]

I recently bought a 2000 Bayliner Capris 1850 bowrider. The dash is an "integrated" type instead of individual gauges--with one exception: there's a behind dash digital depth sounder mounted that displays thru a small square window. It appears to work but the numbers displayed don't make much sense to me. I need instructions on the setup and operation of this sounder. There's a 3-position switch in the helm, momentary-center-momentary operation, that's only marked "Depth".
I have no idea how this is used to configure the meter and the previous owner didn't know anything, either.

I believe this gauge has an alarm that can be used set for encountering shallows. This could be useful.