In the days before specially-designed swim diapers, our parents and grandparents usually swam in the buff or didn’t get to swim at all until toilet training was a done deal. It’s amusing to see the big soggy diapers from the 70s and 80s in old family snapshots, but the kids were probably miserable. And water play should be fun! Nowadays we’ve learned how beneficial baby swim classes can be for bonding and water safety. And even toddlers who are playing at the beach under close supervision can really benefit from wearing a baby swim diaper. But before you race off to buy a pack, make sure you understand just what a baby swim diaper does and doesn’t do. And like most all products marketed to new parents, different brands and styles of these miraculous inventions are not created equal. Here’s how to get the best swim diapers depending on how you’ll use them:

How swim diapers work

Why can’t babies just swim wearing ordinary cloth or disposable diapers? Glad you asked. It’s not just whimsy that makes public swimming pools require parents to put little ones into swim diapers or refrain from using the pool. Here’s why: Regular disposable diapers are designed to absorb mass quantities of urine, but they can’t differentiate between that and pool water. So they’ll absorb so much pool water that your baby’s diaper will get heavy and could cause her to sink. Even if you’re watching every second, this creates a drowning risk. Wearing ordinary diapers in the pool can also be super unsanitary for everyone involved.

Disposable diapers also tend to take on so much water in the pool or ocean that they fall apart, which is no way to hold in poop. Cloth diapers aren’t a solution, either, because they’ll get wet and droop. That can also let feces into the water, which creates a number of negative consequences, though most of them won’t kill anybody. That’s why swim diapers are such a handy invention. They have water-resistant materials that won’t absorb so much pool water. Yes, if your baby pees while wearing one, the urine will leech into the water. A swim diaper doesn’t keep that from happening. But it’s okay for some urine to end up in the chlorinated water, though a bit gross to think about. The reason your baby should always wear a swim diaper in or near water is to keep feces in the diaper and out of the water while your baby swims.

Newborns can’t wear swim diapers

As cool as the whole concept is, babies who are still breast- or bottle-fed can’t do the baby swim diaper thing. Their stools are simply too loose and will get absorbed into the water, even when the newborn is wearing a diaper specially designed for the water. The same for babies (or anyone) with diarrhea. They shouldn’t be swimming in a public pool or even the ocean or a lake. Germs in diarrhea can live for seconds or even weeks in water, even chlorinated water. They also cause various illnesses, some of them quite serious.

Disposable versus reusable swim diapers

Happily, there are reusable swim diapers for parents who spend the summer in or on the water or put a premium on sustainability. But disposables swim diapers are a strong option if you are on a budget or don’t plan to swim that often with the baby. If your baby is in a growth spurt, it may not make sense to invest in reusables that your little water baby will outgrow in a few weeks. Another issue to consider with disposable versus reusable: the disposables tend to sag and leak a little more quickly. On the other hand, when you change the reusable swim diaper you have to flush any solids and tote the swim diaper somewhere to rinse and wash.

The best swim diapers

Lots of public pools sell parents an individual diaper for pool wear, but these aren’t the best option. Instead, you’ll want to make the effort to purchase a diaper that fits your baby snugly instead of hoping the right size is there at the Y or lake beach or wherever the family will be enjoying the water. The whole idea is for the baby to wear a diaper that won’t absorb water and will keep feces contained until you can dispose of it. Some brands offer enhanced features, too. Babyganics disposables, for example, include UPF 50 sun protection and change color to remind you to check for poop and reapply sunscreen elsewhere on the baby.

Keep in mind that similar to adult bathing suits, once the swim diaper is wet it’s tough to take off. Some brands make it a bit easier with tear-off sides, like Pampers Splashers. Some of the reusable swim diapers also have Velcro strips, which can help with both fitting and removal. Other useful features on reusables include snaps on the side and strong elastic bands on the legs to keep the poop in. Avoid any swim diaper that’s designed to be pulled down over your baby’s tush and legs after use–that’s just asking for a mess if your baby poops while wearing it.

And just like any fun outing that involves a baby and his entourage, remember that using swim diapers only works for a short while. Once the baby’s out of the water, you’ll still need a fresh, dry cloth diaper and the other diaper-changing extras. Extra effort? Sure, but the extra enjoyment of playing in the water with the baby is worth every bit of the swim diaper hassle.