Santiago catches a marlin, a fish so large that it pulls him far out to sea before it becomes exhausted. The fish eventually tires and Santiago is able to kill it, but cannot take it into his fishing skiff because of its immense size.
Even if we were two and swamped her to load him and bailed her out, this skiff would never hold him.
He made the fish fast to bow and stern and to the middle thwart. He was so big it was like lashing a much bigger skiff alongside.
(Hemingway, The Old Man and the Sea, Google Books)
The skiff is meant to hold smaller fish, possibly several catches of each but not so many that the skiff founders. The marlin is so enormous -- Santiago estimates over 1,500lbs -- that even if he had help to load it and then bail the water, the skiff would sink. Tying the marlin to the side is the only way he can stay afloat and return home; he can't even cut it up because the whole point is to show the length of this giant fish. Additionally, after Santiago's ordeal, his hands are cut up, bleeding and "mushy" after holding the line, and so he would probably not have been able to load it in the first place.
The fish is bigger than the boat. There is no way to put both the old man and fish in the boat at the same time. Also, the boat is pretty beat up, it might not be able to stay afloat with them both in it.