I have a feeling Hemmingway spent a lot of time out on the ocean. This is not the only book that takes place on a boat. Maybe he thought of the idea when he sat there lonely, trying to catch a fish.
That someone writes from experiences is always evident in a narrative. For, there is a certain affinity for the sea that is conveyed in Hemingway's words in addition to the realistic details such as the responses of Santiago to the movements of the great fish.
Santiago's soliloquies on the boat are not untypical. Many fishermen talk out what they are going to do next as they try different lures, etc.
There isn't a page of this short novel that is teeming with knowledge of the sea and of deep sea fishing. Hemingway was an avid fisherman, and that fact that he can use incredible detail and vivid imagery to have us almost feeling the pain of the whole experience along with Santiago speaks to his masterly of language as well as first-hand experiences in the waters south of Florida.
I agree that Hemingway drew upon his own fishing experience when he wrote Old Man and the Sea. His discussion of the other creatures, such as the birds and the sharks, sounds too personal not to have come from first-hand knowledge. When he talks both about and to the marlin, though, there is clearly a connection based on respect and experience. It is a majestic enemy.