Rash guards are designed for higher intensity water sports and provide a compression fit to protect against the sun and skin irritation from contact with surfboards or other equipment.
Swim shirts, on the other hand, have a looser fit and are primarily designed to protect against UV rays while being comfortable.
Rash guards can come in long sleeve, short sleeve, or ¾ sleeve lengths, while swim shirts typically have short sleeves.
Additionally, rash guards are typically made of a stretchy, quick-drying material, while swim shirts can be made of various materials including cotton.