The accuracy with which your fishfinder detects bottom and other objects is also determined by the frequency selected for the depth you are viewing. Raymarine depth transducers can be tuned to two different frequencies: 200 kHz (high) or 50kHz (low).
200 kHz (high)
200 kHz Echo Sounder Display in 50' (15m) of water
200 kHz works best in water under 200 feet/60 meters and when you need to get an accurate reading while moving at faster speeds. High frequencies give you greater detail to detect very small objects but over a smaller portion of water. High frequencies typically show less noise and fewer undesired echoes while showing better target definition.
50 kHz (low)
50 kHz Echo Sounder Display in 50' (15m) of water
For deep water, 50 kHz is preferred. This is because water absorbs sound waves at a slower rate for low frequencies and the signal can travel farther before becoming too weak to use. The beam angle is wider at low frequencies, meaning the outgoing pulse is spread out more and is better suited for viewing a larger area under the boat. However, this also means less target definition and separation and increased susceptibility to noise. Although low frequencies can see deeper, they may not give you a clear picture of the bottom.
Mud, soft sand, and plant life on the bottom absorb and scatter sound waves, resulting in a thicker bottom image. Rock, coral and hard sand reflect the signal easily and produce a thinner bottom display. This is easier to see using the 50 kHz setting, where the bottom returns are wider.
A rule of thumb would be to use the 200 kHz setting for a detailed view to about 200 feet and then switch to 50 kHz when you want to look deeper. Better yet, display both views side-by-side on a split screen for both perspectives.