Question
what is the deepest water i can plant a lilly in

Answer
What kind of Lilly???  Some Lilies such as the tiger lotus are bulbs that send runners to the top... those can be planted quite deep.  The overall key to planting aquatic plants it to not plant hem so deep that the light can not reach them. I typically do not plant anything that is more then 2.5 feet deep.  For deeper water there are floating plants or plants that will sent off runners with leaves that float.   dave