Most modern depth sounders have an alarm for shallow and for deep water.
Depending on depth contours of the bay you are anchored in, you can set the low end of the alarm appropriately so that it goes off if you drag towards shallow waters (avoid hitting the rocks) AND set the high end accordingly in case you drag and drift out into the ocean.
Very effective and does not rely on any signal.
Will consume some power from the battery, but not that much to worry about. Some older models had a power saving setting for the anchor alarm mode.
I find the depth sounder to be much more reliable.
Octaman
