I always thought they all were, but then their confusion during the episode 'The Bris', during which they agree to become godparents to a couple's newborn -including getting the Mohel and holding the baby for the circumcision - made me think they weren't after all.
George is. Not sure about Kramer. If you remember there was a running joke about Jewish characters falling for Elaine because of her 'shix appeal' (shixa being a non-jewish lady though I've probably spelt it wrong. Therefore I don't think Elaine was Jewish.