Pooja Jal Shankh are real conch-shell, it is having ritual and religious importance in Hindu religion & even in Buddhism. It is the shell of a large predatory sea snail, Turbinella Pyrum, found in the Indian Ocean. In Hindu mythology, the shankh is a sacred emblem of the Hindu preserver god Vishnu. It is still used as a trumpet in Hindu ritual, and in the past was used as a war trumpet. The shankh is praised in Hindu scriptures as a giver of fame, longevity and prosperity, the cleanser of sin and the abode of Lakshmi, who is the goddess of wealth and consort of Vishnu.
The Shankh is considered very sacred in Hinduism and the Shankh is used in almost all the works of worship, but as Hindu mythology Shankh Water should not offer to lord Shiva or Shivlinga. There is a legend behind this, which is forbidden to do so. This is explained in detail in Shivpuran.