Water clarity has no bearing on the unit, nor does day or night (I had someone ask the that saying it wouldn't work at night because the sun isnt out) Sun has nothing to do with it, its just regular sonar waves. PVC does show up but its not very well defind, the shadow will show up better.
The best place for all question and answers is the yahoo group called sideimaging.
Another difference between the 797 and 997, is the 997 uses dual frequency the 797 has only one, most that have the 997 use the one frequency like the 797 has. Also the 797 has only one mmc card slot, the 997 has 2 card slots. If you bought 797 and plan on using a card with lake maps you will not be able to take snap shots with the map card in the unit at the same time. You will have to remove it and place a different card in it.
Here is a pic with mine, it shows some brushpiles and xmas trees the reason you can tell its a xmas tree is look at the shadow it cast.