I have no problem with a basic swimsuit--you know, the kind you see on competitive swimmers or in Azumanga Daioh's swim classes--myself, but for modesty or other reasons some women prefer not to show their bottoms. The swimdress is a good option for those women, as well as being an attractive choice for women who don't have the same concerns. I've liked those flirty little skirted swimdresses ever since I was a kid and I think it's unfortunate they aren't more readily available; they're tasteful, feminine, and more flattering to the average female form than most of the suits out there.
I recently found a tutorial on how to make a swimdress. This swimdress is different from the suits with the little skirt, and is more of a sheath-like dress, worn over a pair of bottoms. It's just as nice, though. The gist of the instructions: Buy a tankini swimsuit plus a duplicate suit in a larger size, cut off the larger tank below the breast and sew the resulting tube of fabric to the other tank.
McCall's #4848 has a swimdress pattern, but it has spaghetti straps and looks no better than the two tankini reconstruction job. And I think you'd have to be either good at sewing knits or very optimistic in your chances of having beginner's luck to attempt a swimsuit from scratch. I'm neither.