Split-shot is often used for shallow water trolling. I would use them in 8 feet or less. In Green Bay it is not always necessary to fish right on the bottom – in fact rarely do we run baits on the bottom because of all the zebra muscles.
I don’t think so much about how deep the water is, but instead how deep are the fish eating. I often start of using all the same size weights whether that is all split shot or all one ounce weights or all two ounce weights, and then I vary the amount of line out to cover different zones in the water column. Once one of the set ups starts getting bites I put more spinners in that zone (replicate the amount of line out). The size of weight you use depends on the depths you want to get to. For 20 foot and under I would use 1 ounce weights. 20 to 40 2 ouncers will work. For shallow water use Split shot. For the deeper water use either Bass Pro Fish Weights (in-line weights) or Offshore Snap Weights (allow you to put out a long leader).