Adding dry ice to a pool causes no harm. Dry ice is frozen carbon dioxide and only produces fog if the water temperature is above 60 degrees Fahrenheit. It temporarily lowers the pH of the water.
Most people want to add dry ice to a pool or other body of water to produce fog. Achieve this effect in a heated pool by adding 50 to 60 pounds of dry ice per hour. In a pool below the required temperature, the dry ice only forms bubbles. In water, it reverts to a gas, forming carboxylic acid and lowering the pH of the pool water. In most cases, exposure to the atmosphere releases the carbon dioxide, so the water returns to a normal pH within a week.