A side imaging fish finder is a device that can be used to scan the ocean floor and visualize objects and organisms below the water. The first thing you want to do is consider what kind of fishing you will be doing. For example, if you are an avid fisherman who spends time on both saltwater and freshwater, then a side imaging fish finder with a depth rating of 300' would be best for you. However, if all of your fishing is done in freshwater lakes or streams, then a depth rating of 50' would work better for you. If your needs fall somewhere in between these two extremes, then a depth rating of 150' could provide enough coverage for both salts.