(1) Background: The aim of this study was to design and analyze the validity of the SMACC (Scale to Measure Aquatic Competence in Children) to evaluate aquatic competence in three- to six-year-old children. In addition, the relation between real competence obtained with the SMACC and perceived aquatic competence was verified as well as its differences according to sex and age. (2) Methods: Content validation was performed through the consensus of nine experts using the Delphi technique, and comprehension validity was determined through a pilot study on a sample of 122 children. An exploratory and confirmatory factor analysis was performed with two independent samples of 384 and 444 school children between three and six years old, respectively. (3) Results: After the pertinent adjustments, the final questionnaire comprised 17 items, which showed a good fit for both comprehension and content validity. The results of the exploratory and confirmatory analyses support the use of three dimensions in aquatic competence: motor, socio-affective, and cognitive. The correlations support construct validity showing a positive relation with perceived aquatic competence. (4) Conclusions: These promising validity data are discussed from a global and integrative perspective in relation to the improvement of children’s development in the aquatic environment during the early stages of their lives.