I think I've found the answer to my own question
"If you only need your sounder to provide basic depth information in mostly shallow water environments there is certainly no need to opt for a pricey top-of-the-line sounder, because it won't be the best fishfinder for you.
Even the least expensive bottom machines on the market today will provide accurate reliable depth information in a graphic and numeric format starting for around $100.
Top notch fish finding sonar units are another story. A top quality recreational fish finder can cost upwards of $1000.
We’ll tell you what to look for to get the best fishfinder for your needs. Several factors weigh in to determine how well a fish finder will display fish, structure, and bottom.
Pixels, More is Better
One of the most important factors to look for when choosing the best fishfinder is vertical pixel count. The more detail you seek from your sounder the more vertical pixels you need.
Screen resolution is usually expressed in a vertical and horizontal pixel count, such as 640 x 480. This means there are 640 dots making up each vertical line on the screen and 480 dots making up each horizontal line.
The higher the vertical pixel count the better the machine will be able to show a distinction between bottom, structure, and fish, especially in deep water.
Here’s why, let’s say you are scanning in 20 feet of water on a unit with 640 pixels of vertical resolution; each pixel represents about a third of an inch of water depth. That means the machine could easily show even a small fish suspended just an inch or two off the bottom.
Now try the same thing in six hundred feet of water and each pixel represents about a foot of depth, as I am sure you can see there is little chance the machine will be able show a separation between fish and bottom.
The horizontal pixel count tells you how much history your screen will show. Remember everything displayed on a fish finder screen is basically history, stuff you have already passed.
If you are looking to use your fish finder for targeting fish in deeper water choose a sounder with high vertical pixel count. For only basic depth information this becomes far less important.
Power, More Might Be Better
If you want your sounder to be able to shed some light on the composition of the bottom under your boat or use it for fish finding in water over a couple hundred feet deep you’ll be best served by choosing a unit with dual frequency capabilities and at least 500 watts RMS power output.
Most recreation depth sounders have between 100 and 1000 watts of RMS power. The definition of RMS power is really irrelevant; we just need to compare units on an even keel. So remember, if a unit states maximum power with a peak-to-peak rating that is simply eight times its RMS power rating. So a machine rated at 4800 watts peak-to-peak is also rated at 600 watts RMS.
The maximum depth capability of a fish finder with 500 watts of output power would be about 500 feet at 200 kHz and somewhere between 1000 and 2000 feet using 50 kHz.
Our fish finder transducer page takes an in-depth look at choosing the right transducer for your marine sonar.
Features to Look For
Here is a basic list of the best fish finder features in order of importance with short descriptions of what each actually does to improve the sounders usefulness.
* Gain – in our opinion, the best fish finders use a rotary knob for manual gain control.
* Bottom Lock – lets you look at a small portion of the water column right above the bottom in an expanded view that makes seeing details easier.
* A-scope – shows in real time what is directly under the transducer. As the machine receives information and writes a new vertical line on the right side of the screen it also shows this information expanded horizontally. The horizontal expansion makes it easier to view this information.
* Zoom – several types can be found on a variety of machines. All basically expand the view of a certain area of the water column to full screen. Again this makes it easier to see details in the area."
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