In Jewish hekhalot mysticism, one who ascends to the heavenly temple may see something which looks like - but is not - water. Should he be deceived by this illusion, he betrays his unworthiness and exposes himself to retribution.
Detailed examination of the water vision discovers that its real object is the celestial pavement, separating the fiery divine realm from the "watery" world of impure organic matter. This pavement is Ezekiel's firmament of hashmal - a luminous crystalline substance - seen by the visionary from above.
Further investigation finds that the water vision continues an ancient tradition of exegesis of Ezekiel 1 as an account of a heavenly ascent, in which "water" signifies materiality, femininity and impurity. The wide and profound influence of these ideas is encountered in a variety of Jewish, Christian and Gnostic sources.