Scuba tanks have extremely dry air inside.
Cold inhibits anti diuretic hormone, causes peripheral vasoconstriction, driving fluid back into the core and stimulating diuresis resulting in losses of plasma volume. The hypercarbia associated with diving decreases anti diuretic hormone, promoting fluid loss from the plasma volume. Diuretics such as alcohol, coffee, tea also contribute to fluid losses, contributing to predive dehydration. This alters stimulation of pressure and volume receptors, leading to further inhibition of ADH and fluid loss. In addition to the above, the state of chronic hypovolemia and hypokalemia caused by oral diuretics is dangerous to divers by increasing their susceptibility to decompression illness and cardiac arrhythmias, respectively.