<p>Antimicrobial resistance (AMR) poses a significant threat to public health and is increasingly recognized within the “One Health” framework, which emphasizes the interconnectedness of human, animal, and environmental health. While extensive research has focused on regulating antibiotic use across healthcare and other sectors, the impact of intensive biocide use on AMR development, particularly in seawater-cooled systems, remains underexplored. In this study, we report the isolation and characterization of a multidrug-resistant <i>Klebsiella quasipneumoniae</i> strain from the cooling water system of a coastal power plant, where continuous chlorination at 0.2 ppm is employed for biofouling control. The isolated strain displayed broad-spectrum resistance to multiple biocides and antibiotics. Interestingly, the strain shown enhances biofilm formation in response to biocides and antibiotics, thereby compounding its resistance profile. Efflux assays with ethidium bromide (EtBr) and whole-genome sequencing revealed that efflux pumps are central to the resistance mechanisms. Additionally, the presence of <i>β</i>-lactamase (OKP-A) and FosA genes confers resistance to the <i>β</i>-lactam and epoxide classes of antibiotics. The strain was found to be salt-tolerant and preferred to grow at normal salinity, indicating a non-marine origin of this isolate. These findings highlight the prevalence of biocide and antibiotic-resistant pathogens in marine cooling water systems that primarily rely on biocides for biofouling control. In line with One Health principles, our research advocates for a reassessment of biocide practices in marine cooling water systems and the implementation of proactive measures to mitigate the spread of antimicrobial resistance (AMR).</p>