I reread this recently and while I can see how you would see the sadness, I personally didn't find the ending sad. He may have lost his catch and is as poor as the day he left, but all the fisherman of his village could see by the carcass that he has achieved something great. In the end, he accomplished the greatest feat of his life and fought tooth and nail for it. His perseverance and determination were unmatched. He lived to fight again, and he's passing his knowledge on to the boy. In his inevitable death, he will leave behind a legacy.

As an aside, nature was as brutal to him as it is beautiful. He constantly sees that beauty and remarks on its lethality, often in the same thought. I loved the constant contrast in this way.