Is it permissible to pray in a swim towel? May Allaah The Almighty Reward you.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If this dress covers the entire ‘Awrah (i.e. the parts of the body that must be covered), is not transparent and is free of impurities, it is permissible to pray in it. Otherwise, it is impermissible because covering the ‘Awrah and purification are prerequisites of prayer. A woman's ‘Awrah in prayer includes her entire body except the face and palms.