Then, why not use this case as a reason to get yourself into a hazmat class?
Me: Park uphill and upwind. Call for additional resources (hazmat, fire). Try to ascertain the nature of the chemical. Does it present a danger to myself or my crew? Does it present a hazard by entering a certain area? What hazards are still presented to the victims? Can they leave the contamination area on their own or do they need to be extracted? Do I have the resources to safely extract them without presenting a hazard to myself or my crew? Can they be safely decontaminated via simple means without presenting new hazards to myself, my crew, the victims or the environment? (Like, can I rinse the chemicals off the victims with a garden hose over a child's sandbox or a tarp or something). Do I have the tools to treat and transport the patient? What equipment needs decontaminated from contact with the patients?
Obviously, many acids react with water, or water might just spread the effects around to harm new areas, so you probably wouldn't use water for your example.