BBC News has an excellent story explaining the history of water-boarding. Unfortunately it has a vast and varying history.

The Khmer Rouge were fast workers. In only a few years they managed to kill one
quarter of Cambodia's population with a mixture of execution and starvation.

But like others who butchered in the pursuit of utopia, they borrowed heavily from the past. Water-boarding was not their invention. The Spanish Inquisition used it and called it the toca.

The Dutch East India Company used it in the massacre of Amboyna in 1623.

For much of history, water-boarding has been to torturers what cream is to apple pie. They go hand-in-hand.

Read more here