This repeated cross-sectional study examined trends in swimming competence and the accuracy of self-assessed swimming ability among ten-year-old children in Osijek-Baranja County (2020–2024). A total of 4340 third-grade students completed both self-assessments and standardized swimming tests each year. Across five years, the prevalence of non-swimmers remained high (64–76%), with significant year-to-year variation (χ²(4) = 29.39, p < 0.05). Persistent discrepancies emerged between self-perceived and actual swimming ability (Fisher’s exact test, p < 0.001). Despite existing initiatives, swimming competence remains low and children tend to overestimate their abilities. These findings highlight the need for stronger school-based swimming education, accurate assessment strategies, and broader community engagement to foster aquatic literacy and improve child safety.