Hi ya Simmo,
Had a brill day on the last day of LBS and also bought Silva S10 and Garmin GPS. It also leaves me to purchase a FF.
Just to be safe I'd keep clear of Garmin for FF's (too much negative feedback man), and choose a different MNFR's. My fav's are Humminbird, Navman, Lowrance and Eagle.
Navman is the best spec.ed, with Log/speed and water temp at quite a resonable price.
I am attracted to the cheap Humminbirds and Lowrance's range though, but I'll make my desision based on the physical size on the unit as I've not a lot of room to fit one.
As for a depth sounder working at speed (over 20knts), I think if you are wrorried about depth and need useful info over this speed then your going too dam fast!!!
Keep me updated on your FF purchase though