Every area is different. I have town water and an irrigation/sprinkler/well pump and both of those options are clear and very cheap. Some places the town water is expensive or the ground water is full of iron and not worth it. Others have a pool too far away from any water sources. All of those people tend to truck water in.
Vinyl needs to have 12 inches of water on the bottom anywhere there is a bottom. (Shallow end if the pool is IG). Plaster and Fiberglass pools need to have water in them to stop the shells from popping out of the ground if the water table is higher than the bottom of the pool shell. If you can dig down that deep and not hit water, You are fine to drain for a short while.
Any holes in the pool..... lights, returns, main drains, skimmers, in floor cleaners, stairs, etc would be easier to work on empty. Unfortunately you would have to know what was needed beforehand which is unlikely on a new to you pool.
