Patrick,
I installed a B&G sonar and I have similar erratic signals.  I think we need to find out why from B&G.  At anchor, it seems to work fine except that it can be confused by reflections from the chain.  While on the way, it can give you a good signal for a while and the start with total garbage.  Now that I know you have the same problem, I will go back and try to work the kinks with B&G.
I agree with Ken that a forward scanner should be very useful in remote places.

Jose
Ipanema SM278
Boston until October, then is the world.