I would definitely go with a dual beam. The single beams are usually 30deg cones, which does not cover much ground in shallower water. I just did the math: if you are in 10ft of water, a 30deg cone gives you a 5ft diameter circle of coverage on the bottom. With a dual beam the second beam is usually 60deg, which will double the diameter.
I just installed a Humminbird 323, and have not used it yet, but I don't like the mount and electrical connections - they do not look like they will be very resistant to salt water, and I already cracked the mount apparently be bending it too far back.