2. For procreation water (or other fluids) are necessary.

"Why have you not given battle? You stand on chariots of water, you are almost turned into water yourself ..." (The Hittites)

"Hylas had been Heracles's minion and darling ever since the death of his father, Theiodamas, king of the Dryopians, whom Heracles had killed when refused the gift of a plough-ox.

Crying 'Hylas! Hylas!', Heracles plunged frantically into the woods and soon met Polyphemus, who reported: 'Alas, I heard Hylas shouting for help; and ran towards his voice. But when I reached Pegae I found no sign of a struggle either with wild beast or with other enemies. There was only his water-pitcher lying abandoned by the pool side.'

Heracles and Polyphemus continued their search all night, and forced every Mysian whom they met to join in it, but to no avail; the fact being that Dryope and her sister-nymphs of Pegae had fallen in love with Hylas, and enticed him to come and live with them in an underwater grotto." (Graves)

3. In the mythical (celestial) frame of reference 'water' is located in the dark.

"In the most general sense, the 'earth' was the ideal plane laid through the ecliptic. The 'dry earth', in a more specific sense, was the ideal plane going through the celestial equator. The equator thus divided two halves of the zodiac which ran on the ecliptic, 23 ½º inclined to the equator, one half being 'dry land' (the northern band of the zodiac, reaching from the vernal to the autumnal equinox), the other representing the 'waters below' the equinoctial plane (the southern arc of the zodiac, reaching from the autumnal equinox, via the winter solstice, to the vernal equinox. The terms 'vernal equinox', 'winter solstice', etc., are used intentionally because myth deals with time, periods of time which correspond to angular measures, and not with tracts in space." (Hamlet's Mill)