Objectives <p>Motor impairments affect about 80% of children with autism spectrum disorders (ASD), making aquatic interventions potentially beneficial for motor development. To better tailor interventions, the Wot-ASD, an observation instrument for water-related motor skills in children with ASD, was developed. In a pilot study, feasibility, reliability, and validity were evaluated.</p> Methods <p>The first stage of development involved mastering an existing instrument based on the Halliwick program. In stage 2, items were operationalised based on observed behaviours. Stage 3 validated the content with feedback from six experts.</p> <p>Stages 4 and 5 established scalability, interrater reliability, internal consistency and validity. A convenience sample was utilised to provide a preliminary psychometric evaluation of the Wot-ASD. The instrument’s feasibility was evaluated across all sub-studies.</p> Results <p>The Wot-ASD was developed with four subscales, each corresponding to a learning phase, to assess 25 items covering 22 activities. Expert feedback supported the validity of the Wot-ASD, noting that it accurately and comprehensively assesses water adjustment skills. Preliminary results showed adequate internal consistency for all items (ICC &gt; .81), and scale consistency with H ≥ .5 for both items and subscales, and interrater reliability was supported by a Cronbach’s Alpha &gt; .91 for 23 items.</p> Conclusions <p>This pilot evaluation of the Wot-ASD provides preliminary evidence for the feasible, reliable, and content validity when evaluating water-related motor skills in children with ASD.</p>