Most of the vintage-style suits on the racks now have a few things in common: They provide a fair amount of coverage (especially in the rear), the leg is low-cut and they've got some glamour going on. Different decades, though, do offer their own features. For instance:
With the lowest-cut leg, a '30s-style swimsuit is almost tunic-length, often with an attached skirt. A self-belt might add some interest at the waistline.
A retro-'40s suit has a low-cut leg without the skirt; it might have a sexy halter top and strategically placed ruching, which is great for covering up imperfections. It may also be a two-piece, revealing a few inches above the navel.
In the '50s, corset-style boning came on the scene. A mid-century suit can provide the shape you weren't born with or accentuate what you've already got. It can also stay put in the ocean.
Bikini time! That means a two-piece revealing some skin below the navel, still with a low-cut leg (but higher than decades past). It's probably a print, and it may very well have yellow polka dots.
"Retro," of course, is not the same as "vintage." Modern twists on classic looks can offer welcome updates. Some retro styles feature beading, crystal accents, cutouts or military details that bring a decades-old look into the present.
The suit, though, need only be the beginning. There's so much more to retro swimwear.