Although water-based approaches have been shown to be beneficial for children with Autism Spectrum Disorder (ASD), no study thus far has directly investigated the effects of such intervention programs on language skills. The present study aims to evaluate the efficacy of the Aquatic Speech and Language Therapy (ASLT) program, which is a new, exclusively aquatic intervention program designed especially for children with ASD. The effects of ASLT were compared to the outcome of a similar classroom-based intervention, in two groups of children with ASD matched for age, gender, and expressive/receptive vocabulary. Our findings show that ASLT results in significantly greater improvement of vocabulary measures, thus providing direct evidence of water-based intervention's beneficial effects on language skills in ASD.