Drowning is a global health problem that can be addressed with multiple strategies including utilisation of lifeguards in recreational swim areas. However, few studies have described lifeguard prevention activities. We conducted a retrospective analysis using lifeguard activity data collected in real time with a Computer-Aided-Dispatch (CAD) system to characterise the nature of lifeguard primary and secondary drowning prevention at a popular ocean beach in California. Preventative actions constituted the majority (232 065/423 071; 54.8%) of lifeguard activities, while rescues represented 1.9%. Most preventative actions and rescues occurred during summer months, weekends and afternoons. Statistically significant geographical clusters of preventative actions were identified all over the beach, while rescue clusters were primarily restricted to two sites. Using the most reliable and valid collection system to date, these data show spatial and temporal patterns for ocean lifeguard provision of primary prevention as well as secondary drowning prevention (rescue).