In France, to prevent drowning accidents in public swimming pools (PSPs), bathing must be constantly supervised by qualified staff. However, fatal drowning regularly occurs in supervised aquatic facilities. A review of the literature shows that human supervision is a complex task. The aim of this research is to fully assess the periods during which supervision is not carried out, or carried out in an inadequate manner. The observations made in 108 French PSPs show that supervision is not carried out 18% of the time and that it is carried out inadequately 33% of the time. The medical literature shows that, in order to expect to survive without after-effects, an immersed victim requires intervention within a time limit of not more than three minutes; however, we noted, over a total observation time of 54 hours, 147 periods (29.8%) during which the supervision system was degraded for three minutes or more. This quantification research on the periods of degraded supervision is complemented by an identification of the causes leading to these degradations, from which we can draw interesting areas for improvement, particularly from an organizational point of view, in order to improve safety management in French PSPs.