The one quality, more than any other quality, that makes Santiago such a good fisherman is perseverance. He simply doesn't know how to give up. He has been fishing his entire life, and that in and of itself shows readers that Santiago isn't the kind of person to try out one career after another when things start to get tough. This is especially evident at the point in which readers meet Santiago. He has been fishing more than 80 straight days with nothing to show for it, yet he goes out day after day. His fight with the marlin also shows how Santiago is willing to persevere and endure great hardships to be victorious.
Another quality that makes Santiago a great fisherman is his knowledge of the trade. This comes from his lifetime of being a fisherman, and it is why he is able to not lose hope over his current dry spell. Finally, like any other good fisherman, Santiago is patient. Personally, I would like fishing if it meant that I was actually catching stuff more than I was fishing. This relates to the "May" comment. Fishing in May, when the weather is good and the fish are biting, is easy. A true fisherman goes out, fishes, and catches the fish in September. Santiago is patient, and he perseveres at all times of the year.