Thanks so much for your in-depth responses Snakehead Whisperer! I had tried to find a manual for the Bottomline SF without success, so that was extremely helpful. Looks like the Bottomline unit does indeed do what I was looking for - detecting fish between bot and shore. Given that fact, an old unit may be a workable option. I am a bit concerned about things like the age of the unit (I don't know how old they are) and their reliability. Obviously there would be no warranty. I don't mean to put you on the spot, but given my intended use (float tube) would you go with an older Bottom Line unit or would you opt for a new Piranhamax color or Lawrence Elite-4? No pressure, and no blame-game later. I'm just trying to decide what would be best for my son.
Thanks a ton,
Steve