Rebecca's death is perhaps the saddest of them all, as she doesn't see it coming and it's certainly not her choice. Everyone is led to believe she dies by suicide by drowning due to the supposed betrayal of Peter Quint. In actuality, Peter reveals his ghost to Rebecca and in the weeks leading up to her death, periodically takes control of her body so they can be together in her memories.
Peter tells Rebecca that in order to be together forever, she has to fully let his ghost into her body. While she believes they'll equally be in control, it's Peter that takes complete and total control. As Rebecca, Peter walks into the water and drowns her body so that their ghosts can be together. By the time Rebecca realises what's happening, it's too late and she is dead.