The eighth edition of the Armada de Rouen, a major international gathering of sailboats and military vessels, took place from June 8 to 18, 2023, attracting more than six million visitors. The nature of this highly popular event led Santé publique France to strengthen its health surveillance system for the occasion. Surveillance was conducted using the Surveillance des urgences et des décès (SurSaUD®) reporting system, supplemented by an event-specific system based on first-aid stations deployed on-site. On-site health surveillance made it possible to collect and categorize 1,639 visits to first-aid stations and 53 emergency room visits related to the event. The leading cause of these visits was “feeling unwell,” likely due to the extreme heat. The number of emergency department visits related to a visit to the Armada de Rouen site remained relatively low compared to the number of people treated by first responders at the Armada de Rouen site. Monitoring the activity of on-site first-aid stations therefore appears to be central to the system for identifying potential unusual health incidents. Furthermore, this system protects emergency services in the Rouen metropolitan area from an increase in their workload.