I use the bottom. If I can see it, then it is "visible," and then depth at which I can no longer see it is the "visibility" depth. I gauge this depth by either standing in it, or by sticking my wading stick or rod tip in it. If you want to get serious you can make a turbidity tube or a simple weighted secchi disc (high contrast black and white design) with a calibrated line. When you can no longer distinguish the black and white design, to have reached the visibility depth (secchi depth). Johnny's different coloured pebbles serves the same purpose.