Don White wrote:

I'm thinking..for coastal cruising/daysailing I'm really only concerned
with accuracy between 0 and 100 feet max.

I'll add my support for mounting inside the hull - I had no trouble
seeing down 100+ feet shooting through a sold hull. Being plumb helps
but is not essential (within limits).

And I can't let it pass without mentioning, a fishfinder is vastly
superior to a simple digital sounder, and often even cheaper. Even if
you never plan to fish, its really helpful to have a history of your
depth. Some of them will give a good reading of the nature of the
bottom for anchoring.