I purchased a Humminbird HDR650 in 2013. Installation was simple, and I'm satisfied with its performance. I mounted the transducer in the hull next to the drive shaft packing box. Depth readings are accurate below ~28 mph. I paid $100 from Overton's or iboats, can't remember which.

It has a shallow depth alarm, and a keel offset which makes me more comfortable allowing others to take the boat into the beach.

The transducers do not work through air. When installing the transducer, use a slow setting epoxy to allow the air bubbles to work their way out before the epoxy sets.