Because I'm so proud of my MSPaint photoshopping, and because the other thread got muddled, I thought I would post this here
There are three ways I can think of to do a swimsuit cosplay.
1) Use an existing swimsuit design for a character, taken from a show or artbook. (such as using Chii's swimsuit from the beach episode of Chobits)
2) Make a full existing costume out of swimable materials. (Jo from Burst Angel's outfit would make an excellent swimsuit.)
3) Modify a costume into a swimsuit variant (See below)

I'll be using Mahoro to show the possibilities! Isn't she cute!
Type one: Taking a costume straight from the show.This is Mahoro's actual swimsuit from the show:

All the cosplayer would have to do is match the hair and buy a green swimsuit.
Type Two: Making an existing costume into a swimsuit.
This is Combat-ready Mahoro.
It would take some work, but using the right materials a talented cosplayer could make this swimsuit to be much like a wet suit. She would have to take care to use swimsuit-type materials though so it would be quick drying and not get to heavy when wet. I would also recommend not wearing the boots. You could also make the arm armor into water wings for bonus points!
Type Three: Modifying a normal cosplay design into swimwear.
Here is a swim version of her maid outfit.

And here is a two piece version of the combat outfit!
Does this clear things up?