Melt the butter in a large saucepan over a low heat, add the onion and celery and cook, stirring
frequently, for 5 minutes, or until softened but not browned.
Add the potatoes, carrots, water, salt and pepper. Bring to the boil, then reduce the heat and
simmer for 10 minutes, or until the vegetables are tender. Add the fish to the chowder and cook for
a further 10 minutes.
Pour in the milk and heat gently. Taste and adjust the seasoning, if necessary. Serve hot.