BigCat.
Try this, I had the X15 when they were first released and had been updating it ever since. But you can do this without updating.
Go to Sonar features and select "surface clarity" press enter and the select "high". This will show most surface clutter very high on your display leaving a better viewing screen. Then you can adjust the sensitivity higher showing more detail and better bottom display. In most cases more sensitivity the better. However too much and it's a lot harder to pick out individual fish from patches of bait.
Loosing picture can come from many reasons. When entering shallow water any aeration from the hull, motor etc will loose picture. I have had my unit reading in 1 foot of water, which showed it could do it but if you couldn't see fish in 1 foot of water the I would need a "Braille" screen to read it ;P
It most case I like to leave the unit in Auto depth range, that is unless I am only interested in the bottom or mid water, you can also set your depth range for the first 10 feet or whatever if you looking for the fish you described. However the bottom will not be viewable.
I will say one thing you have a brilliant fishing tool in your X15c, and in time you will become dangerous with it, and your mates will think your on drugs when you will see things they can't
One thing I used to do was record the sonar data for the day and then play it back on my PC, because you PC is colour and the recording is done in colour, YOU WILL SEE 100% more on your PC than on the monochrome unit.(More pixels you see)
No longer a Lowrance Rep
Hope this helps
Rob