Check out the www.humminbird.com/productlines_wide.asp?ID=616" Matrix series by Humminbird. I've had great luck with mine. I think it's a 17, but they change the number every year. It's a dual beam 20 + 60 cones. When I fish the Mackerel schools in ~60' of water you can see the school of macks and the Bass in it. Fish in the 60 degree cone show lighter and fish in the 20 cone show solid. You can also set it to return the actual arches and it's got a decent interface with enough filters to get it tweaked out and working without returning many false readings.
Sometimes I use it to find fish but mostly I use it to find structure and basically keep from hitting structure.
Hmmm. seems the units listed on humminbird's site don't really match up to the 17, although that 3d unit looks cool. Here's another link showing the 17 maybe it's discontinued. www.davesmarineelectronics.com/index.asp?PageAction=VIEWPROD&ProdID=5394" I've had great luck with Broken leg Dave Not to be confused with broken leg daves.com