Why couldn't Santiago ever get comfy in the boat? :3

Santiago is old and stiff. He must balance his body on the small skiff. He could also never get comfy because he had to remain vigilant in his battle with the marlin lest he fall asleep. Santiago's body also begins to fail him as the battle wears on, "What kind of hand is that," Santiago says, "Cramp then if you want. Make yourself into a claw. It will do you no good".