Is is possible to have a thermocline this time of year and on the size of lake I was fishing?  I went out yesterday on a small lake, about 23-30 acres that is about 30ft deep at it deepest.  As soon as I would get into water deeper than 12 feet there was a solid line accrose my depth finder at about 11-12 ft.  Would this be a thermocline?  Water temps were about 54 degrees with air temps in the low 60's.

If I am correct the thermocline usually occurs in the summer...but I may be wrong. From what I understand it shows as a wide band streaming across the width of your sonar screen (if you have a LCD).

Something like this:

I'm not sure if it is a thermocline, but I have seen that same thing on just about every body of water I have been on this year and it is usually right at 12' in the same water temp range.

Not real likely that's what you are seeing although the depth can be at that level for certain.  The lakes are still stratified (mixed at all levels) as the hot and cold air have not done thier thing yet which creates an oxygen rich level in the water that happens to show up on some electronics.  

See if you have anything depth related signal wise turned on or that might have been set at the factory.  That may be what you are seeing.  Thermocline this early with air and water temps as close as you are seeing is unlikely.