Both incidents are examples of crossings, so to speak. Denver was born on the Ohio River when Sethe was escaping from the Sweet Home plantation in Kentucky to Cincinnati, Ohio--a free state. Denver is the product of her mother's crossing to freedom. Similarly, Beloved emerges from the water. She crosses from the land of the dead to the land of the living. Just before Beloved's arrival, Sethe feels something happening to her body--almost as if her water is breaking during childbirth. Beloved's arrival, therefore, is connected to a birth.
But if Denver's birth represents the future, Beloved's birth represents a return to the past. Denver's birth is hope; Beloved's arrival presents a chance for atonement for Sethe's killing her daughter rather than have her be returned to slavery. Sethe must be able to deal with each in order to fully heal.