"Shallows and Amazons"

from Wiki and Wiki:

The film opens with a young boy kicking a soccer ball along the shore of a beach. He finds a GoPro camera attached to a helmet washed up on the surf which he picks up. The video on the GoPro is of a man struggling to get out of the water and onto a reef only to be dragged under. The film then relates the outdoor adventures and play of two families of children. These involve sailing, camping, fishing, exploration, piracy and being killed by an enormous shark.

The Walker children (John, Susan, Titty and Roger) are staying at a farm near a lake in the Lake District of England, during the school holidays. While waiting for a wave, Nancy notices a large humpback whale carcass floating about twenty yards away. They sail a borrowed dinghy named Swallow and meet the Blackett children, Peggy who sails a dinghy named Amazon and Nancy, who prefers surfing. While riding a wave back to the beach, a large great white shark bumps her surf board, causing her to fall and hit her head on a submerged rock formation.

The shark, normally an ally of his nieces, has withdrawn from their company in order to write his memoirs, and has become decidedly unfriendly. As Nancy surfaces to her board, the shark bites her leg, pulling her under. Nancy swims to an isolated reef, where she places a tourniquet on her leg to stop the bleeding. When the children meet, they agree to join forces against their common enemy - the Great White Shark whom they call "Captain Flint" (after the character in Treasure Island) The children depart, unaware of Nancy's situation, leaving Nancy to spend the night on the reef.

In order to determine who should be the overall leader in their campaign against Captain Flint, Peggy Blackett and the Walkers have a contest to see who can capture the others' boat. Several hours later, the Walkers return. They get into the water before Nancy can warn them away, and two of them are killed by the shark. As part of their strategy the surviving Walker children make a dangerous crossing of the lake by night, and John is later cautioned by his mother for this reckless act.

Nancy retrieves one of their GoPro camera helmets, and after another close attack from the shark, records a message for her father and sister, and then throws the helmet toward the shore. The Walkers finally  win the contest - thanks to Titty who seizes the Amazon when Peggy Blackett comes to Wild Cat Island.