Instruments can help you sail your boat more effectively, and may add to safety. The most obvious safety item is a depth sounder. Knowing how much water is under your boat is critical, and the trend is also important,
One of the most popular instruments is the Raymarine TriData. It provides depth, water temperature, boat speed and distance log in a single easy to read display. It can also interface with other instruments through SeatalkNG or NMEA 2000 networks. Click on the image for more details.
My own boat has separate instruments for Depth, Speed, and Wind Speed/Direction. They are Raymarine ST60 models, now out of production. The modern replacements are the i50/160 series.