Drowning is one of the leading causes of death in children and teenagers. Individuals with autism spectrum disorder (ASD) are at increased risk for drowning. Improvements in swim skills have been observed in children with ASD participating in learn-to-swim programs. However, it is unclear if age, co-occurring conditions, and/or the dose of practice influence swim skills in this population. To this end, a secondary data analysis of iCan Swim program data was conducted to determine the efficacy of the 5-day adapted learn-to-swim program for a cohort of children with ASD ages 3–16 years (n = 86). Participant swim level was evaluated at the start and end of the program. Linear mixed-effects regression was used to examine the effects of Time (start/end), Age, Dose of Swim Practice (i.e., total time–time out of the water), and ADHD status on the overall swim level. Participants significantly increased the swim level from the beginning to the end of the program (B = 0.63, 95% CI = 0.52–0.74), and participants with ASD and co-occurring ADHD had greater swim levels regardless of Time than those without ADHD (B = 0.45, 95% CI = 0.05–0.84). Overall, iCan Swim is effective in improving the swim skills of children and teenagers with ASD.