Santiago has been fishing on the ocean for as long as he can remember; he, like the other fishermen in the village, takes the younger boys out onto the ocean to teach them how to fish. The teaching is less about the mechanics of fishing than it is about the feelings and instinct; Santiago has the specific instinct and can pass that along to Manolin. It is implied through the text that Santiago is respected but not very well liked, and that he is a loner; not implied, but outright stated in the text, is the love that Manolin and Santiago feel for each other.
The old man had taught the boy to fish and the boy loved him.
He only dreamed of places now and of the lions on the beach. They played like young cats in the dusk and he loved them as he loved the boy. He never dreamed about the boy.
(Hemingway, The Old Man and the Sea, Google Books)
This can be seen in the way that Manolin takes care of Santiago, bringing him food and coffee, and making life easier for him by taking care of many things that Santiago might forget at his age. Manolin also believes in Santiago's innate ability to fish the ocean, even at that age, and at the end of the book, he is sad to see how badly Santiago is hurt. Their relationship is that of a mentor to a protégé, but for both, the connection is deeper; neither will admit it because of their masculine refusal of emotion, but they are more of a father and son.