Urban natural spaces are increasingly promoted as a strategy to strengthen resilience against chronic lifestyle-related diseases associated with urbanization. Among these, urban blue spaces have gained attention for their mitigative, restorative, and instorative health benefits, making their design for health promotion a growing priority in public health and urban planning. This study investigates outdoor gyms (OGs) and their role in promoting physical activity in coastal San Diego, USA. Users’ behavior was examined by employing the System for Observing Play and Recreation in Communities and the Blue Health Behavioral Assessment Tool. Supplementary, an online survey was used to further examine visitors’ perceptions towards the OGs of our study area and which design characteristics might affect their use. Systematic observation showed that OGs accounted for 28% of vigorous physical activity onsite, suggesting they function as a co-equal component of the physical activity affordance mix. Community members viewed the gyms as cost-effective and valuable additions to the blue space, while noting that design characteristics influence their use. Ocean views, proximity to amenities, and the availability of accessible equipment emerged as key enabling factors. These findings offer actionable insights for local stakeholders and underscore the importance of community co-design in realizing the health potential of urban blue spaces.