In a large, deep skillet, cook chorizo over high heat for 2 to 3 minutes, without adding any extra fat to the skillet. Add potatoes, tomatoes, and water, and bring to a boil. 

Reduce heat to low, cover, and simmer for 12 minutes or until potatoes are tender.

Add cod, season generously with pepper, and continue cooking with lid on over low heat until fish is opaque and flakes away easily with a fork (about 2 minutes), or until a meat thermometer inserted in thickest part reads 63°C (145°F).

Break cod up into chunks while in skillet and serve.