Mix coconut milk, tomatoes with juices, onion, garlic, ginger, curry powder, cumin, and salt in a 4-quart slow cooker.
Cover; cook on high 2 1/2 to 3 hours until onion is tender and sauce hot throughout.
During the final hour of sauce cooking, bring rice and 1 1/2 cups water to a boil in a covered saucepan. Reduce to low; cook 40 to 45 minutes. Rest covered 10 minutes.
Nestle cod into hot sauce. Cover; cook on high 20 to 30 minutes until fish reaches 145°F and flakes easily. Avoid breaking pieces by stirring.
Fold spinach around fish; cover 3 to 5 minutes until wilted. Gently stir lime juice into sauce.
Divide rice among 4 bowls; lift fish, spinach, and sauce over it with a broad spoon.