I coaxed my DD into getting her face wet in the bath in a relaxed setting instead of a high sensory overload and stressor environment like a swimming pool. Baby steps, confidence building and an emphasis on fun helped my DD.

Another thing that I think adds to the stress of an indoor pool environment, at least, are the acoustics - lots of very loud echoing voices and cries. This isn't going to help a child with sensory issues feel relaxed at all.

Become what you are