How does a fish finder work? I fully understand sonar and how it works but I cannot fathom(no pun) how it's displayed. When you look at the screen is it just a cross-section of the water directly below you or does it show sort of an inverted funnel seen from the top down?

2d sonar is a cone figuring the exactness is a trigonometric function and I don't remember which one it is.  Generally speaking at 200 kHz the cone widens out until it reaches the bottom of the water column.  The bottom of the cone has a diameter of approximately 1/3 of the depth so in say 15 feet of water you would be seeing a circle with a diameter of about 5'.  Down scan is a whole different animal.

This should help