He was an old man who fished alone in a skiff in the Gulf Stream and he had gone eighty-four days now without a fish.
The opening line of Hemingway's novella clearly sets the stage for all the disadvantages that Santiago holds against la mer, "which is what people call the sea in Spanish when they love her." For, his is a small fishing boat and he is aged and nearly defeated. Thus, Santiago is extremely vulnerable on the vast and mighty ocean.
After having caught the marlin, Santiago considers the prowess of this creature--"Man is not much besides the birds and beasts"--unless sharks come. This puissant predator is capable of the worst damage because not only could he eat Santiago's fish, but he could easily overturn the small boat which could knock Santiago as he is thrown from it and injure or possibly kill him. Of course, if he is in the water with the shark, conscious or not, Santiago could easily lose his life.