The Halliwick concept is widely used in different settings to promote joyful movement in water and swimming. To assess the swimming skills and progression of an individual swimmer, a valid and reliable measure should be used. The Halliwick-concept-based Swimming with Independent Measure (SWIM) was introduced for this purpose. We aimed to determine its content validity and inter-rater reliability. Fifty-four healthy children, 3.5-11 years old, from a mainstream swimming program participated in a content validity study. They were evaluated with SWIM and the national evaluation system of swimming abilities (classifying children into seven categories). To study the inter-rater reliability of SWIM, we included 37 children and youth from a Halliwick swimming program, aged 7-22 years, who were evaluated by two Halliwick instructors independently. The average SWIM score differed between national evaluation system categories and followed the expected order (P<0.001), whereby a ceiling effect was observed in the higher categories. High inter-rater reliability was found for all 11 SWIM items. The lowest reliability was observed for item G (sagittal rotation), although the estimates were still above 0.9. As expected, the highest reliability was observed for the total score (intraclass correlation 0.996). The validity of SWIM with respect to the national evaluation system of swimming abilities is high until the point where a swimmer is well adapted to water and already able to learn some swimming techniques. The inter-rater reliability of SWIM is very high; thus, we believe that SWIM can be used in further research and practice to follow the progress of swimmers.