Hero Water is a special liquid available in the kingdom of Arabasta.
The Hero Water gives whoever drinks it superhuman strength. The drinkers can easily be recognized by seeing strange markings on their arms (veils). However, any individual would die within five minutes after consumption, ultimately making it a desperate last resort.
The Hero Water was used by the Tsumegeri Guards of Arabasta in an attempt to defeat Crocodile, knowing full well that their lives would be forfeit in five minutes. Crocodile easily evaded their enhanced powers with his intangible Suna Suna no Mi powers, and being cruel and ruthless, refused to even fight them to give them an honorable death, mocking them for their stupidity when they succumbed to the poison.
A liquid with the same name and a similar effect is seen in the Naruto series OVA Mission: Protect the Waterfall Village!