Rescuers were called to the Delaware River shore in National Park in Gloucester County late Sunday afternoon.
There were reports of numerous people in the water.
Three men were able to get out, but a 44-year-old woman went under and did not resurface.
The Coast Guard says the woman was aboard an inflatable raft in the Delaware River and was reportedly not wearing a life jacket.
Investigators now believe she died, and the search for her body is underway.
There is no word yet on how this accident happened.
