Heat oil in a 5- to 6-quart pan over medium-high heat; add onion and garlic and stir often until onion is very limp, 8 to 10 minutes. Add tomatoes, 1 1/2 cups water, potatoes, chilies, cumin, chili powder, and salt; increase heat to high and bring to a boil. Reduce heat, cover, and simmer until potatoes are tender when pierced, 20 to 25 minutes.
Stir in seafood, if using. Cover and simmer until shrimp and fish are opaque but still moist-looking in the center of thickest part (cut to test), 3 to 4 minutes. Stir in cilantro. Spoon quinoa onto rimmed plates; top with stew.
Nutrition analysis per serving of seafood stew.