I can't get my head around how the 25=BA beam works, given that the mix
of data from the bottom of the cone at any one point is displayed on
the screen as only a single data entry. Let me explain - if the depth
of the water at the extreme right side of the cone is, say, 100ft, and
the depth at the extreme left is 120ft, what does the screen show? OK,
I can understand that if there's a fish anywhere in the beam, that will
appear on the screen, but what about varying bottom data?

It would "think" the water is 100 ft deep ... the FIRST echo reply it
gets is the depth ... so wide beam transducers will not see in holes
that are narrower as the beam ...

You can download garmin manuals on their webpage which explain a little
bit how it works ..

and get a dual frequency transducers ...

I cant confirm that only 200 khz sees fish ... I doubt it but havent
seen many fish on my finder ..

Matt