The Blue Lagoon

In the Victorian period, two young cousins, Richard (Glenn Kohan) and Emmeline Lestrange (Elva Josephson), and a galley cook, Paddy Button (Leo McKern), survive a shipwreck in the South Pacific and reach a lush tropical island. Paddy cares for the small children and forbids them by "law" from going to the other side of the island, as he had found remains from bloody human sacrifices. He also warns them against eating a scarlet berry which is apparently deadly. (He calls it the "never-wake-up berry".)
Paddy soon dies after a drunken binge, and his body is discovered by Richard and Emmeline. Now alone, the children go to another part of the island and rebuild their home.
Years pass and they both grow into tall, strong, and beautiful teenagers. They live in their hut, spending their days together fishing, swimming, and diving for pearls. Richard and Emmeline (now portrayed by Christopher Atkins and Brooke Shields) begin to fall in love, although this is emotionally stressful for them because of their lack of education on human sexuality. Emmeline is frightened after she begins her first menstrual period; confused about it, she refuses to allow Richard to inspect her for what he imagines is a cut.
Sometime later, their relationship suffers a major blow when a ship appears for the first time in years. Richard's desire to leave comes into conflict with Emmeline's desire to stay, and she does not light the signal fire. As a result, the ship passes by without noticing them. Richard's fury leads him to kick Emmeline out of their hut. They make up for this fight after Emmeline is nearly killed upon stepping on a stonefish and Richard admits to his fear of losing her. Emmeline recovers and after she regains her ability to walk, they go skinny dipping in the lagoon and then swim to shore. Still naked, Richard and Emmeline discover sexual intercourse and passionate love. They regularly make love from then on while occasionally spending their time together in the nude. Due to their regular sexual encounters, Emmeline becomes pregnant. During the pregnancy, Emmaline discovers cravings and morning sickness as she continually indulges in coconut and fruit then is discovered by Richard the following morning vomiting by the seashore. Richard and Emmeline themselves do not know about the truth of childbirth and human reproduction and assume that the physical change in Emmeline's body is her getting fat. They are stunned when they feel the baby move inside her and assume that it is her stomach causing the movements.
One night, Emmeline gives birth to a baby boy, whom they name Paddy. Frustrated at not knowing how to feed the baby, Emmeline holds him and learns how to feed him as the baby instinctively starts sucking on her breast. The young parents spend their time playing with Paddy as he grows, teaching him how to swim, fish, and build things.
As the family plays, a ship led by Richard's father Arthur (William Daniels) approaches the island and sees the family playing on the shore. As they are covered in mud, their appearance is difficult to determine. When they notice the approaching ship, and are currently happy with their life on the beautiful island with the young Paddy, they exchange looks and tacitly to walk away and stay on the island instead of signaling for help. Arthur assumes that these are natives, not the young couple they have been searching for all these years, and the ship passes.
One day, the young family takes the lifeboat to visit their original homesite. Richard goes off and finds bananas for them to eat, leaving Emmeline and Paddy at the boat. Emmeline looks around the shore of the island and does not notice when Paddy brings a branch of the scarlet berries into the boat. Emmeline and Paddy return to the boat and slowly drift away, until Paddy tosses one of the oars out. Unable to reach the oar, Emmeline yells to Richard and he swims to her, followed closely by a shark. Emmeline throws the other oar at the shark, striking it and giving Richard time to get into the boat. Though close to shore, they are unable to return or retrieve the oars without risking a shark attack. They paddle with their hands to no avail; the boat is caught in the current and drifts out to sea.
After drifting for days in the boat, Richard and Emmeline awake to find Paddy eating the berries he picked. Realizing that these are poisonous berries, they try to stop him, but he has already swallowed a few (and they do not know how to induce vomiting). Hopeless, Richard and Emmeline eat the berries as well, lying down to await death. A few hours later, Arthur's ship finds them floating in the boat. Arthur asks, "Are they dead?" and the ship's captain (Alan Hopgood) answers, "No, sir. They're asleep".

Question: What did Richard and Emmeline realize about the berries?
Answer: they are poisonous berries