When wearing a swim shirt or rash guard, the best choice is either a bikini top or a sports-bra style swim top made from chlorine-resistant, quick-drying fabric. Traditional bras are generally discouraged because their cotton or foam padding absorbs excessive water, becoming heavy and saggy, and the underwires or metal clasps can corrode in salt water or pool chemicals. Furthermore, a regular bra won't dry quickly, which can lead to skin irritation or "chafing" during a long day at the beach. In 2026, many specialized "athleisure" brands offer high-impact swim bras designed specifically to provide support under rash guards for surfing or paddleboarding. If your swim shirt has a "built-in" shelf bra, you may not need an extra layer at all, but for those requiring more support, a racerback bikini top is ideal because it stays hidden under the high neckline of most swim shirts and allows for a full range of motion. Avoid anything with bows, beads, or bulky hardware that might create visible lumps or discomfort under the tight-fitting fabric of the shirt.