Actually tried something interesting out yesterday. We bought one of those Humminbird Smartcast jobbies that you tie on like a lure and toss it out to where you want to check depth/temp/fish. Transponder looks like a little green submarine.
But I'm conflicted over the thing, so far.
First, it seems somewhat useful. Considering what most people pay for fish/depth finders, $80 plus tax seems pretty decent. And we used it on Castaic yesterday and it, I think, was useful. I say "I think" because it seemed pretty consistent about readings and most of the info was more or less observationally verifyable, plus, it caused me to abandon an area that looked 'perfect' but which registered absolutely no fish and we went to an area where we hauled in two nice ones. "I think" it helped me make the decision to do that, and if it wasn't just a random thing based on false readings, it paid off.
But it's not very durable. At least I don't think so. It failed after about an hour in the water and 30-40 casts, but the storem took it back so I'm going to give it another try. Worst-case, I return it again. I have to say though, if it works out, it's pretty dang convenient to eyeball a patch of water, decide it looks pretty, and toss the bouy 50 feet straight to it without moving the boat and get instand depth, temp, contour. The display leaves a lot to be desired, though. It's about the size of most of the avatars on this forum, and very low rez. But as was said, it's how you use the basic info, maybe not the super-detail of the screen. Heck, my cell-phone has WAY more detail. Then again, it cost WAY more. I sure like being able to carry both the transponder and the display in my shirt pocket, though.
I'm kinda looking at the Garmin 140, too. Think I could make that more or less portable with some minor modifications.