I have one made by Humminbird. . .I've had it for 4 or 5 years. Honestly it's not very useful.
It comes with a screen like you'd have in the boat and a wireless transducer. You tie the transducer on a rod and cast it. When it his the water it starts transmitting.
It's only real use it looking at depth and temp. It's GREAT for that. But the sonar isn't good enough to accurately make out fish. Sometimes you can see them, sometimes you can't. Usually can't.
If you're fishing a new area from the bank it's nice to be able to see the bottom contours so easily, without having to drag a t rig or a c rig around the area to learn it. It can help you eliminate water for sure.
But it's really not going to help much with finding fish under a dock. Honestly you might be better off getting one of the cheaper underwater cameras for that.
This is similar to the one I have, but this one doesn't have a wireless transducer.http://www.basspro.com/Humminbird-PiranhaMax-197C-Portable-Fishfinder/product/1412051043/
Here's an example of the underwater cameras I was talking about, in case you aren't familiar.http://www.basspro.com/AquaVu-Micro-II-Underwater-Camera/product/1305301139/
Good luck buddy.