Diabetes insipidus is due to the deficiency of

Anti-diuretic hormone (ADH) stimulates the reabsorption of water by the distal tubules and thereby reduces loss of water through urine (diuresis). An impairment affecting synthesis or release of ADH results in a diminished ability of the kidney to conserve water leading to water loss and dehydration. This condition is known as diabetes insipidus. 

Hyposecretion of insulin causes diabetes mellitus.