Why do they flush people with water after chemical attack?
A standard protocol after unintended chemical exposure is to quickly douse the victim in large quantities of water to wash away chemicals that could remain and cause further harm, such as powders and other types of solids that could spread over your body as you moved. Also, the water will dilute any chemicals to the point of least harm that soak into clothing (provide a large amount of water is used). However, as Ivan Neretin pointed out in the comment, sarin gas does not do physical damage to the external parts of the body, but enters the body and causes nerve damage extremely quickly. In this type of situation, the water dousing probably did little to nothing to help the victims.