This isn’t a preposterous book. It’s theme is simple: it tells a story of a man that goes fishing when a marlin bites his bait and he ends up on the battle of his life.
The book is beautifully written. Yeah, maybe not everybody will be hooked by the simplicity of this story. And there is no symbolism, as the own Ernest Hemingway said once:
“There isn’t any symbolism. The sea is the sea. The old man is an old man. (…) The sharks are all sharks no better and no worse. All the symbolism that people say is shit. What goes beyond is what you see beyond when you know.”
Despite the lack of symbolism, the story teaches you a lesson or two about perseverance (or even stubbornness) and by doing the things that really matter to you, not mattering what the other people will think about it.