Also make sure you can OFFSET the reading, too. . . . you want
to be
able to calibrate in the depth difference offset from the transducer's
location to the bottom of the keel.

Two schools of thought here; the "make it read 0 when the keel touches"
and the "make it read the actual depth, 0 equals no water." The
lattter seems more common, easier to correlate the depth to chart
soundings and what other folks say the depth is. To each his own
though.

Any depth sounder that will read to 50' is deep enough. . . .
Who cares if the
depth is 3000' or 300'?

Beg to differ, a good deep-reading depth sounder is a very useful
navigational aide. Can be used to determine how far offshore you are,
pick up undersea canyons which lead to harbors and anchorages,
establish a single LOP, etc.. An example: Consulting a chart reveals
there are no onshore hazards (pinnacles, reefs) outside the 100 fathom
line of a particular shore. Using the depthsounder, one can follow the
100 fathom contour line and be assured of no nasty surprises on a
coastal passage.