Combine water, tomatoes, jalapeno peppers, onion, tomatillos, serrano peppers, and garlic in a large pot. Bring to a boil. Reduce heat and cook until vegetables are fork-tender, about 30 minutes.
Transfer the vegetables to a blender with a slotted spoon, allowing the cooking liquid to drain back into the pot. Cover the blender and hold the lid down with a potholder; pulse a few times to combine. Blend until smooth, 1 to 2 minutes. Season sauce with salt and pepper.