Just guessing here, but if the same "noise" is on the screen when the PS21 is out of the water, then the PS21 is definitely bad. There should be nothing on the screen when it is out of the water.
I can post a screen shot of how mine looks out of the water later. Its mostly blank with a very small red fuzzy area very near the transducer location.
The reason for the 2D shot is to see if the 2D is picking up the same "stuff" in the water that is showing on the PS21 screen. Thats to see if there is something under the boat causing the problem - a weed bed, etc.
Those two things could prove there is a problem for sure.