Although in Europe the quality of swimming pools (SPs) is dictated by regulations, microbiological and chemical hazards are described in the literature. Environmental bacteria or toxic disinfection by-product (DBP) compounds may indeed be recovered in waters even after disinfection. We evaluated the water quality from 26 outdoor seasonal SPs of the Versilia district, according to requirements of Regional Decree 54R/2015. In spring 2017, supply and reinstatement waters were collected after shock hyperchlorination (10 mg/L) while in summertime, a second sampling of waters before entering the pools, as well as in the pools, was performed after SPs were open to the public. In all samples, microbiological and chemical parameters were determined as defined by Directive 98/83/EC and the Italian Health Ministry. Microbiological data were within suggested limits. The first chemical analyses showed that in 35% of the feeding-pool seawater samples, the halogenated organic compounds were higher than the maximum permissible concentrations (30 μg/L). Pool waters were then dechlorinated and re-treated with hydrogen peroxide (10 mg/L) to ensure the abatement of DBPs (from 164 ± 107 to 0.9 ± 0.8 μg/L; p = 0.002). Results highlighted the need of self-controlled procedures for the SPs waters to prevent waterborne diseases and suggested hydrogen peroxide as the most appropriate disinfection method.