In a medium saucepan, bring the water and stock to a boil. Add the potatoes and onion, cover and simmer over low heat for 10 minutes. Add the carrots, celery and jalapeño, cover and simmer for 8 minutes. Stir in the tomatoes and bok choy. Season the fish fillets with salt and pepper and lay them on the vegetables. Cover and simmer for 2 minutes. Carefully turn the fish fillets and simmer uncovered until just cooked through, 1 to 2 minutes longer. Using a slotted spatula, carefully transfer the fish fillets to shallow bowls. Season the stew with salt and pepper and add the basil. Ladle the stew over the fish and serve.