Fishfinder

I have a fishing buddy 2201. They use a clamp like a rod rest at the side of the boat and then they just slip onto it, very simple and no messing around with batteries or wires. It also has the side finder. Now they are called fishfinders but in all honesty unless you are fishing in 50ft + all the time any fish that it shows up will be spooked by the boat(i use it to troll for ferox). They will tell you the dept and give you a good idea of the bottom but its up to the angler to interrupt that info and to catch the fish. Some people have a mis conception that they are like some window into the fishes world and you can simply drop baits in fron of there noses

I have the side scanning and I'm pretty sure it works as described, when dapping I would see a fish symbol the same distance that the fly is working, and a lot of fish come up to the dap that you never see. I also tested it on released fish and it did register the fish moving away from the boat. However I dont think it ever helped catch a fish
At 20 ft what you see depends on the quality of the finder and what width the cone is at 20ft. As I use mine for trolling it tells me the dept, type of bottom. over an area of probably 20square foot. The deeper you are the wider the cone
This is the style I have, http://www.cabelas.com/product/Boating/Boating-Electronics/Sonar-and-SonarGPS-Combos%7C/pc/104794380/c/104707080/sc/104588280/Humminbird-Fishin-Buddies/735123.uts?destination=%2Fcatalog%2Fbrowse%2Fboating-boating-electronics-sonar-and-sonar-gps-combos%2F_%2FN-1100517%2FNs-CATEGORY_SEQ_104588280%3FWTz_l%3DSBC%253BMMcat104793480%253Bcat104707080&WTz_l=SBC%3BMMcat104793480%3Bcat104707080%3Bcat104588280

They are handy for small boats if you are hiring the boats, however they wont stay in the water at speed. If you own the boat and your handy at the DIY I would get the installed type.