Swim diapers are designed to contain any bowel movements created in them. They aren't designed to hold urine, whether the swim diaper is a disposable or cloth. Furthermore, they are designed with this intent in mind while covers and empty pockets are not. So in short, no; they aren't the same and you shouldn't use an empty pocket or a cover in place of a swim diaper.
So if they aren't designed to catch pee, should you put something inside to catch it? No, you don't have to. I put an infant prefold inside my son's swim diaper because he wears the swim diaper on the way to the pool or the local water park. I don't want to have a wet car seat or worse, a wet shirt from carrying him to the pool. Yuck! You could also solve this problem by changing your baby into their swim stuff just before entering the pool.
This brings us to the pool regulations. Most pools and water parks do not allow you to change your baby's diaper poolside as it increases the risk of contaminating the water closeby. Aside from that, all that is discussed in regards to swim diapers is that they contain the poop. There is no mention of whether it must be cloth or disposable. You can check out the CDC website listed below for their complete list of do's and don't's when swimming with a baby.
And here's something you should know: the chlorine in the pool can cause serious damage to your covers and empty pockets over time rendering them useless in the future. Additionally, many swim diaper companies recommend against using a swim diaper as a cover whether they'd make a good cover or not. If you wish to do this, you should keep the swim diaper for the pool and the swim diaper for the cover separate so as to prevent cross contamination.
Finally, I have to say after using both disposable swim diapers and cloth swim diapers, I don't think that the disposables are better. I actually think they're worse since you're spending about $10 for just one pack of 10-12 disposables that can only be used one time. If you're like us, you do a lot of swimming in the summer. We could go through two packs of disposable swim diapers in just one month! That's enough money to buy a cloth swim diaper that we could use over and over again! I think we'll stick with our cloth swim diapers.
1. Applecheeks- http://www.applecheeks.ca/site/en/What_Swim_Diapers.html
2. Wee Little Changes- http://weelittlechanges.com/item_157/Applecheeks-Swim-Diapers.htm 3. Huggies Little Swimmers- http://www.littleswimmers.com/na/products/swimpantsfaq.asp
4. My Swim Baby- http://www.myswimbaby.com/catalog.php?category=187
5. Centers for Disease Control and Prevention (CDC)- http://www.cdc.gov/healthywater/swimming/pools/swim-diapers-swim-pants.html