Grill bratwurst, covered, over medium heat, turning frequently, until meat is no longer pink, 15-20 minutes. Grill onion, covered, until lightly browned, 3-4 minutes on each side. Cut bratwurst into 2-in. pieces.
In a stockpot, combine 2 qt. water, beer and seafood seasoning; add corn, potatoes, lemon, bratwurst and onion. Bring to a boil. Reduce heat; simmer, uncovered, until potatoes are tender, 15-20 minutes. Stir in cod; cook until fish flakes easily with a fork, 4-6 minutes. Drain; transfer to a large serving bowl. Sprinkle with pepper.