I am putting a small pool in our basement. Would it be better to use pressure treated plywood as ceiling, or sheet rock?

Pressure-treated plywood is treated with chemicals to resist rot and decay, making it unsuitable for use in a pool environment, where it would be constantly exposed to moisture.

Sheetrock (gypsum board) is a type of drywall made of gypsum plaster pressed between two sheets of paper. It is not water-resistant and would quickly become damaged if exposed to the high humidity and moisture levels of a pool environment.

For a pool ceiling, you should use a water-resistant material such as PVC (polyvinyl chloride) panels, fiberglass reinforced panels, or aluminum panels. These materials are specifically designed to withstand moisture and humidity, making them ideal for pool environments.