I have an older signet depthsounder, with the transducer behind the keel, and a Lowrance dual frequency fishfinder/gps with an airmar transducer in front of the keel. When both are on I see a series of short small horizontal lines moving up the display of the fishfinder as it scrolls, clearly interference, although it is just a minor inconvenience. The Signet only really works in shallower water (less than 75 feet) anyway, otherwise it likes to read 18.8. I'm not sure if having the FF/GPS on makes this worse or not.
Why have them both? Because I find it easier sometimes to see the Signet from anywhere in the cockpit while the Lowrance is mounted at the helm. Also it is sometime important to know what the depth is under different parts of the boat in very shallow water.
Ross - my backup is the forward scanning lead based keel. What the boat goes bump, it's a shade under eight feet deep. Now I know why its called 'Canoe Passage'.

Warning... this technique is usually followed by some fiberglass repair.
But the bobber suggestion very interesting. My other alternative is to give a kid my polarized glasses and send them forward. That finds rocks that are just a few feet down, but if one is 6 or seven feet under, they don't always see it. I could give them a rod and keep them busy.