If you're looking for anime similar to The Mysterious Cities of Gold, you might like these titles.
After the death of his father, the young Jim Hawkins has lived a quiet life helping his mother run their Admiral Benbow Inn; but he and his mother’s life change forever when the drunken scoundrel Billy Bones arrives at their door. Through a series of harrowing events, Billy dies, leaving Jim a treasure map that leads to the famed fortune of the dreaded Captain Flint; and with the help of two local gentlemen the trio (along with Jim’s feline friend) set out to find a ship and crew to achieve their goals. In the nearby town of Bristol they find the one-legged John Silver who joins their ship, as well as several others. However, as the journey begins, Jim soon realizes that things may not be as they seem…
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The child protagonists in these series are about the same age when they embark on a ship to find a long lost treasure. They both form a close relationship with an experienced sailor. If you enjoyed the mentor-protégé connection between Esteban and Mendoza, then you will most likely feel the same about Jim and Silver's in Takarajima. Similar to Mendoza, John Silver is a charismatic yet cunning leader and father figure to the young boy while pursuing his own agenda. He will do anything it takes to find the gold. Both series are old-school children's anime that have adult characters in prominent roles.