Drop shotting shallow water can be very difficult and ineffective from my experience. Thus I consider shallow water to be 1 1/2' or less.

I would consider my optimal range to be 3 1/2' to 7' of water.

A major dependency for me is if you will be fishing from a boat. I find it extremly difficult to drop shot from shore or docks.

*** Mind you, this is purely based on my own experiences, from the Northern waters of Michigan. Good luck, Tight Lines!