Isn't this "ワンピースは実在する！" from this anime/manga? It's not related to dresses or swimsuits.
In this manga, ワンピース is a name of "ultimate treasure" searched by pirates, and very little is known about it. Characters in the manga even do not know whether ワンピース is a touchable object; it may be a name of some magical power, or a city, and it may be just a baseless rumor.
So in this context, "ワンピースは実在する" means "One Piece does exist in reality. It's not a mere legend or rumor." The fansub movie above translates this as "One Piece does exist," which I think is perfectly correct, but I find nothing wrong if you translate this as "One Piece is real."
Besides that context, of course, "(something) is real" cannot always be translated as "実在する". If you want to say something like "This one-piece dress is a real one manufactured in Italy in 1850," use 本物, not 実在する.