Because hydrogen leaks frequently ignite, and because about half the time the ignition source is not identified, when evaluating hazards with hydrogen leaks, many people just assume the leak will be ignited. Note that consideration needs to be made for what may happen with immediate ignition (jet fire) and what may happen with delayed ignition (explosion). 

It is still important to minimize the probability of ignition and to minimize the consequences if it does ignite. We do that by properly characterizing hydrogen with all its unique properties, including flammability and low MIE, and by providing the appropriate safeguards prescribed by codes and standards specific to hydrogen. These safeguards include minimum quantities, using proper materials of construction, leak prevention practices, proper ventilation, proper disposal to safe areas, and ignition source control such as the use of non-sparking, electrical grounding, and classified electrical equipment.

There are many potential sources of delayed ignition. Hydrogen is easily ignited, and the larger the cloud, the more likely it is for it to find an ignition source. The cloud itself may serve as an ignition source as the force of the gas release may cause dust or other contaminants to mix in the air creating a static charge which ignites the hydrogen. Similarly, the force of the release can cause surrounding materials and equipment to create an ignition source from impingement. Examples include doors being pushed against surrounding equipment, crushed stone being blown against equipment leading to an impact and spark, etc. The release may also be larger than the hazard assessment or codes anticipated, thereby extending the vapor cloud into areas with other ignition sources such as unclassified electrical equipment, hazardous activity by personnel, or surrounding equipment. 

The oxygen in sprayed water would not be a hazard for delayed ignition

Delayed ignition is a significant hazard for hydrogen releases, either intended or unintended. The primary concern is the overpressure and energy release created from a vapor cloud which could contain a significant quantity of hydrogen. The H2Tools Incidents database contains a number of examples of delayed ignition.