Fabric
Main fabric: Four-way stretch knit swimwear fabric with at least 50% stretch in both directions. Look for a mix of nylon or polyester with elastane (AKA Lycra/Spandex). Lining: Swimwear lining, stretch mesh or more of your main fabric, with at least 50% stretch in both directions.
Notions
All versions: 8-10mm wide rubber swimwear elastic
Bikini version: 6mm wide rubber swimwear elastic
Optional: Foam swimwear cups