See that swimsuit? Study it closely. No, keep looking. Okay, now, notice anything weird about it? If you said "well gee it kinda looks compostable," you're much better at observing the finer points of swimsuit technology than I am, because guess what, you're right: it is compostable. And so is the skimpier version.
Yep, when I look at those swimsuits, "dirty" is the absolute first thing that pops into my head. You know, like compost. Eco-luxury (?) designer Linda Loudermilk created these designs using a special fabric made from plant starch, and while you can wear it just like a normal swimsuit (out in the sun all day, in the water, etc.), it'll break down completely within 180 days if you bury it in your backyard. It's also supposed to be cheap enough to be practically disposable, which is handy for an article of clothing that disposes in such a friendly way.
Since the swimsuits are designer pieces that just debuted at Miami Swim Week, it's probably going to be a while before they're something you'll be able to dig up at your local swimwear shop. Until then, just remember that the most environmentally friendly swimwear is no swimwear at all.