Yep, I hear that it's toxic in large quantities.

The dangers of dihydrogen monoxide include:

• Also called ‘hydroxyl acid’, the substance is a major component of acid rain
• Contributes to soil erosion
• Contributes to the greenhouse effect
• Accelerates corrosion and breakdown of electrical equipment
• Excessive ingestion may cause various unpleasant effects
• Prolonged contact with its solid form results in severe tissue damage
• Inhalation, even in small quantities, may cause death
• Its gaseous form may cause severe burns
• It has been found in the tumors of terminal cancer patients
• Withdrawal by those addicted to the substance causes certain death within 168 hours

Nevertheless, governments and corporations continue using it widely, heedless of its grave dangers.

source: wikipedia