A fishfinder is the angler's eye under the water. In this group you will find fishfinders and chartplotters, as well as combination units that show both on the same screen. You will also find transducers for extending what your unit can do.
Start with screen size and how you fish. A small fishfinder is enough for keeping an eye on depth and fish, but a chartplotter with a larger screen makes navigation and route planning much easier. A combination unit shows the chart and sonar side by side, which is handy on open water.
The transducer decides what you see on the screen. A traditional transducer looks straight down, down imaging and side imaging transducers draw an almost photographic picture of the bottom, and live sonar shows the movements of fish and lure in real time. Always check that the transducer is compatible with your display.
The mounting position affects image quality. A transom mount is the most common solution, and it needs to sit where water flows smoothly past without bubbles, or the picture will break up at speed. Save productive spots as waypoints, so you can find them again with ease.