Again, you need to select this product with your personal fishing in mind. Choose the model that has the options you require. The ability to add speed, temperature, and a trip log require that you purchase a middle to high-ranged priced depth finder. As you step up in price, with both Humminbird and Lowrance, you also get units with more pixels. These are the little squares that pass along the screen from left to right to build the image.
More pixels bring sharper detail to the screen.
Another option, or feature, on the higher dollar units are dual and tri-beam transducers. These allow you to see three to five times what you could see with single crystal transducers. The companies usually offer suggestions for best positioning of the transducer with an emphasis on placing it outside of the hull. As long as the boat doesn't have a second floor that will work at all but the highest speeds. The position is critical and either a dealer who knows where the transducer should be placed should seat it for you or you will have to move it around until you the find the best read.