Objective: The aim of this study was to assess lifeguards’ knowledge retention of airway management, oxygen administration, and ventilation interventions following certification and employer-provided training. Methods: This cross-sectional study was conducted using an online survey administered between February and May 2024. A total of 1322 responses from Canadian lifeguards certified in airway management and oxygen administration were deemed eligible for analysis. The survey included 15 knowledge assessment questions, with data analyzed based on lifeguard experience and the date of last certification or in-service training. Results: The mean knowledge assessment score was 10.4 ± 2.2 (69.3 ± 14.6%), with the highest scores in the airway management category and the lowest in the oxygen administration category. Lifeguard experience significantly increased knowledge retention, whereas recertification showed no significant impact, and employer-provided training significantly decreased knowledge retention. Conclusions: The findings underscore the importance of lifeguarding experience in knowledge retention among lifeguards. Optional airway management and oxygen administration recertification, coupled with inconsistent in-service training, have created significant gaps in lifeguard education. This study identifies the need for regular, competency-based training delivered by qualified facilitators. Addressing these gaps is crucial for enhancing the effectiveness of lifeguards in emergency response and ensuring high-quality care for drowning victims.