They use a scrim that's slanted 45 degrees. The actor lies on it, while other actors in the scene stand behind it. The rest is lighting. There's a great description and photo in George Perry's coffee table book for the show.
It really was a phenomenal effect. It made the audience feel like they were at the bottom of the pool looking up at the body floating over them. Joe Gillis entered with a group of reporters and photographers snapping pictures like they just left OJ's crime scene, and Gillis was left alone onstage beginning the narrative.
Doesn't look like an actor on a scrim which is tilted fwd about 45 degrees. Looks more like a projection or the figure being painted on the scrim to me.