A complete fishfinder / sonar system consists of:
Some multifunction displays require a separate DSM and GPS Antenna while others have these built into the display. Choose from a range of high-performance MFD's by Raymarine.
A powerful sonar system can be achieved by networking a sonar module to a Multifunction Display via SeaTalkhs. Greater detail can be revealed by using a sonar module with CHIRP technology.
For additional features such as to drop a waypoint you will require a GPS. Your MFD may already have a GPS, but if not you can easily add a Raymarine RayStar130 to your network.
Finally, you will need a transducer. The type of transducer required depends on the multifunction display you're using, your vessel (type of hull) and the performance you require.