Santiago is an old man who has spent many hours on the sea trying to catch enough to make a living. That much time in the sun and sea have taken a toll on his looks. We first learn that,
"The old man was thin and gaunt with deep wrinklies in the back of his neck." (pg 9)
Because he was out in the sun so much, he was exposed to skin cancer. People, however, were not as concerned about it in 1952 as we are today. However, Hemingway appeared to be aware of it. He says,
"The brown blotches of the benevolent skin cancer the sun brings from its reflection on the tropic sea were on his cheeks. The blotches ran well down the sides of his face and his hands had the deep-creased scars from handling heavy fish on the cords. (pg 10)
It is interesting that Hemingway describes the skin cancer as "benevolent" or kind. Much of his physical description as it is given in the book is due to Santiago's occupation. Sun and sea weather the skin and increase the amount of wrinkles. The sun causes brown blotches. However, there is one thing the sun did not damage.
"Everything about him was old except his eyes, and they were the same color as the sea and were cheerful and undefeated." (pg 10)