Santiago loves everything about being a fisherman. He has great respect for the sea as well as the creatures that inhabit it. He does not, of course, love every one of the creatures of the deep (as we see when he battles the sharks); he does, however, have respect for them.
We know he respects these things because he understands them and knows their habits intimately. He knows the taste and qualities of tuna; he knows each kind of shark by name and description; he knows the patterns of a fish which has been hooked; he knows the signs of the weather and the tides. Even in his worst moments, being dragged out to sea and waiting interminably for the fish to surface, he is never angry at the creation for doing what it was created to do.
The most compelling element of the novel which demonstrates Santiago's love for these things is the fact that he calls this great fish his brother. Though he is on a mission to catch (and thereby kill) this fish, he treats it in his mind with dignity and respect. And, since he calls it brother, he must feel as if he, too, is somehow like that fish.
Santiago has lived a long time being good at what he does; the fish, too, has lived a long time to have evaded capture and grown to such a size. The fish is free, but he is also a creature of habit--just like Santiago. They are both destined for this fate: Santiago to fish and his brother to be caught.
It's clear Santiago is a man who is more at home on the water than in his actual home. Both are places which provide little in the way of comfort, yet he is more alive and happy when on the water seeking his elusive brothers.