Consider setting a depth alarm on your sounder. Pick numbers a couple
of feet less than you will have at the lowest point in your intended
anchorage cirlce at the lowest tide, and a few feet deeper than you
will experience at the predicted high. Assuming you are going to anchor
in the shelter of an island or a bay, (rather than in the middle of an
undersea plateau), depths should vary quickly enough outside the
intended circle to alert you that you are moving.

Yes, this is an excellent method too, but not for all bottom profiles. The
GPS covers the other cases well