even if it is only a Depth System, you will need something. Most of the systems are fairly self explanitory in their function, and a Depth System measures water depth.
A Wind Instrument, predominantly found on sailing boats, measures wind speed and direction relative to the boat. In this mode, it is known as "Apparent Wind". If you interface Boat Speed data to the wind system, the wind system can then display actual wind speed and direction, known as "True Wind". Although on a Motor Boat, a wind system is less important, they can be extremely useful when mooring .
Speed/Log systems measure boat speed through the water. A "requirement" for the keen yachtsman who wants to get the best from his sailing and his Wind Instrument.
Using a single brand for your suite of instruments tends to give best results due to interfacing compatibility.