Maybe a Frigate (1980)

The major force in this story is the children - the group of mischievous girls and boys who can play all the time. They find an antique amphora at the bottom of the sea. Since there is an amphora, there must be a frigate. There is a mystery cave in which the captain goes empty handed and comes out with full packs. That is how the search begins. The children and the adults are passionate about it. Instead of a frigate, they find a truth: "We do not need much to have fun."