This pattern is designed to work with stretch/knit fabrics, particularly spandex.
For swim shirts, rash guards, compression shirts, athletic shirts, and other performance-oriented shirts, four-way stretch fabric is strongly recommended. For more casual raglan t-shirts, two-way stretch jersey should work fine.
For compression clothes and swimwear, a 20:80 ratio of nylon/polyester to spandex/elastane is recommended. More elastic fabrics allow for greater negative ease, will give better mobility, and will be more forgiving to measurement/design errors.
The raglan sleeves lend themselves well to using different colors/prints for the body and for the sleeves.
With swim fabric/spandex, it is recommended to make the neckband out of the same material used for the shirt. For less stretchy fabric, such as cotton jersey knit, ribbing is recommended.