You don't need any special accessories to swim with your bag. I think those wraps are meant to hold your bag closer to your body so it isn't as noticeable in your bathing suit, if you're concerned with that sort of thing. If you are worried about
your wafer sticking as well after getting very wet, it shouldn't be a problem, but there are extra adhesives you can get to help ensure a good seal. These are two that I like... alpglobal.com/Sure-Seal-TM-Rings.html
You can order free samples of either one, the first one MIGHT be a better option but I have used both and they both work very well.
That said, I have been to the beach a couple times since surgery, and splashed around in the ocean a good bit. At that time I didn't have any special accessories or extra adhesive protection and it didn't change the wear time of my wafer at all. I also don't think my bag was visible through my suit, and I'm a female, so my suit bottoms are pretty tight.
DX:6/12 Mild UP:got/kept remission w/1 Canasa nightly
MAJOR Flare began end of 11/12
Flex Sig 12/14/12:UC,mod/severe left side
Flex Sig 2/4/13:UC Severe into transverse colon
Prednisone refractory,little to no relief from enemas or Remicade
Proctocolectomy w/end ileostomy 3/13/13
Pathology showed Severe Chronic UC into right colon
Finally off Pred as of 4/24/13
So far my new,healthy life is GREAT!