My exact same problem a couple years ago.
Contact Airmar @
http://www.airmar.com and tell them what you got and what you need. They fixed me up a P66 unit with a "low Q" value and on my Carolina Skiff, you can read bottom till you hit it. Down side on the Skiff, won't NOTHING work over 24-25 MPH. I'm using a Garmin 240 and keep it on salt water readings all the time. Other than the speed issue, I'm very happy with them both. +/- $100 for the ducer. Can't help on the FF. I'm trying to make up my mind on a new color unit myself but, If you go Garmin, don't go BLUE nothing. It's too strong for the shallows.