Water lilies' containers must be about 18 inches wide and 10 inches deep. Plants with shorter stems and smaller root systems can grow in smaller containers. Soil - Forget what you've learned about potting soils. Avoid regular potting mixes and soil amendments. They contain elements that will rot, pollute, or float.
Aquatics grow best in containers filled with ordinary garden soil.
The heavy clay garden soil that you regularly curse over is fine for aquatic plants. Dig some up, and break up clods to use it as aquatic potting soil. Some people cover the soil with sand or pebbles to keep it from muddying the water and to prevent fish like koi from digging up the plants. After planting, simply soak the soil in the container and then set the container in the pool. It's heavy enough not to float, and the water clears in about a day.