All fishfinders use sonar which is a sound that moves down through the water in the shape of a funnel. The deeper the water the bigger the funnel the small end is your fishfinder and when the sound hits an object like a fish or submerged log it bounces back and shows on your screen along with the depth.
If your fishfinder has GPS be sure to get as big a screen as possible since the GPS will be sharing it.
Beginners should purchase simple fishfinders and then advance as they need to. For most people LCD fishfinders are a great choice CRT displays are excellent but will require you to carry another battery as it eats power.
All fishfinders have KHZ frequency and there are many choices to choose from. Most of these are made for freshwater those with lower KHZ are used mainly in saltwater. Mid to high range KHZ will show the contour of the bottom in great detail.
Also make sure you get a transducer with your fishfinders some may not have them those that use a single frequency are for freshwater and those that have a dual frequency are for saltwater.
They have portable fish finders that are mainly used in small boats. Also color units are much easier to read in direct sunlight the greater the resolution the easier it will be to see.
Fishfinders
