Rather than learn the hard way, which is often the most costly or most embarrassing route, remember the following tips when choosing a suit for swim practice.
Don't wear oversize swimwear.
Unless you're looking for extra drag, do not wear oversize swimwear. When you purchase swimwear, make sure it fits well. Suits that are purchased a size too big are only going to get bigger with time and wear. Not only is oversize swimwear uncomfortable and unsupportive, it also tends to wear out sooner.
Don't wear padded cycling/triathlon apparel.
Padded gear absorbs water, creating unwanted drag and potential chafing. Plus, most cycling and tri apparel is not designed for extensive use in chemically treated swimming pools and will wear out prematurely. Save triathlon apparel for its intended uses: cycling, short distance swimming or as a base layer to a wetsuit.
Don't wear gear with front zippers or pockets.
Front zippers can easily scratch you or others who may come in contact with you. And, while zippers may help you get your suit off more easily, they also have a tendency to unzip on their own, causing accidental exposure. Gear that has pockets that do not close may fill with water to create unwanted drag.
Do wear swimwear that is properly sized.
For women, your practice suit should fit comfortably on the shoulders and across the back, without feeling short in the torso or small on the backside. If you can pull the suit straps above your ears, the suit is too big. If you are unsure of sizing, use your chest measurement as a general guideline. Dress sizes and additional body measurements can aid in proper sizing.
Men typically have an easier time sizing suits, as swimwear sizing usually matches waist sizing. Men's practice suits should be easy to tie and should feel comfortable at the legs. When between suit sizes, I encourage swimmers to choose the smaller size, as swimsuits tend to stretch with age and use.
Do wear swimwear that provides adequate coverage.
Not only should suit fabric cover you appropriately, it should be completely opaque. Transparency due to poor color choice, fading, rips, or holes should be avoided at all costs. To play it safe and get more wear out of your suit, I recommend purchasing dark colored swimsuits with proper lining or even double-layered swimwear. For those that are comfortable wearing more than one suit at a time, layering swimsuits is a great option during swim practice.
Do save your fashion swimwear for the beach.
Non-endurance suits that are designed for leisure are not made to withstand chlorine, physical activity, edge of the pool sitting, and sun exposure associated with swim practice. In addition, fashion suits sometimes feature metal clasps, jewels or other decorative accessories that may interfere with comfort or break during your swim.