A movie about a little boy who adopts a water monster but then lets it go back to the sea in the end

The Loch Ness Monster (2007 film directed by Jay Russell)

Anne MacMorrow is a single mother, and Angus MacMorrow is a boy who is afraid of water but loves water. His father's whereabouts during the war are unknown, and Angus and his mother have been living in a village. The village is close to the sea. He often went to the beach to play and look for shells in the puddles with bumpy stones on the beach. Once Angus accidentally saw a very large egg in a puddle, so he took it home and hid it in the store. in the cabinet in the room. A miracle happened the next day, the egg hatched, and Angus was astonished when a strange reptile appeared in front of him. He named the little animal "Crusoe". Although Crusoe was naughty, he was very obedient to Angus's words. Angus regarded Crusoe as his friend and took good care of him. Although he was happy, Angus also felt a little worried. But Crusoe grew bigger and bigger, and he was afraid that his mother would ask him to give up Crusoe. At this time, their big house was requisitioned by the army as a shelter, and Angus became more careful. Fortunately, with the help of Lewis Mowbray, they put the growing Crusoe into Loch Ness. Angus often ran to play with Crusoe and dived underwater for the first time under Crusoe's leadership. The army living in Angus's house mistakenly thought that the huge Crusoe was an enemy submarine and attacked the poor creature. Angus desperately rescued him, and Crusoe took him and swam away from the shore. When the boat that rescued Angus swam closer, he realized that it was Crusoe. But the officers and soldiers on the shore could no longer be notified, so they had to rescue Angus on the boat first. The officers on the shore hoisted the net high and tried to stop Crusoe, who was mistaken for an enemy submarine. Crusoe jumped over the net and escaped. Angus returned to the shore and saw Crusoe looking in the distance. He looked at Crusoe and smiled slightly. Decades later, the old Angus told people about Crusoe in a restaurant. At the end of the film, a boy picked up the same egg on the beach.