Most of us fish inshore in water 25 deep or less, more often than not in water less than 6 feet deep. A lot of the features in the more expensive FF's have limited use in shallow water. Almost any FF can tell you how deep the water is and if there is any break or structure down there in that situation.I find it hard to justify spendinghundreds of $$ on an expensive FF whan an more economical one will do the job. Remember, you will still need to buy a battery, battery holder, waterproof storage, and mount - all things that will run up the cost. What will be your power source? The less expensive units can run all day on 8 AA batteries. The more powerful models may need more juice. If you fish in the gulf a lot, than perhaps a more capable unit makes sense - dual beams, more resolution, bigger screen, etc. I will let others speak to that.
If you are in 6 feet of water and your FF has a 60 degree cone, which is common - that means it is seeing 30 degrees on either side of the boat. Using basic trigonometry - no I don't have that in my head, I looked it up - the FF is seeing out 3.5 feet on both sides of the yak or a circle 7 feet across on the bottom. Probably not much use as a fish finder, but very useful for finding depth and any breaks in the bottom. Now if you are in 30 feet of water, it is seeing a much bigger cone of water.
A good tip is to buy the most pixels in the vertical part of the display that you can afford - more pixels in the vertical mean the picture will have that much more resolution - more pixels per inch of water column... The Eagle Cuda 128 has 128 pixels in the vertical, the Eagle Cuda 168 has 168 pixels and the Eagle Cuda 242 has - guess - 242 pixels in the vertical....The 168 is $67 at Academy, the 242 is $87. Humminbird, Garmin and other brands put out good models in that price range. I have two Eagle Cudas, they work just fine and run all day on 8 AA bateries.
I have the Humminbird watch / remote thingy - it came with the yak. It works OK, but is a little inconvenient to use. There is the hassleof tying it on and using up fishable casts.I used it once or twice and now it sits in the closet...
Check out the FF install articles in the rigging section of the GCKFA home page.