Here's the quote:
"The only completely stationary object in the room was an enormous couch on which two young women were buoyed up as though upon an anchored raft. They were both in white, and their dresses were rippling and fluttering as if they had just been blown back in after a short flight around the house."
The women are "buoyed up" like a raft on the Sound, highlighting the connection to the sea and the vastness of the landscape.