Why are Narwhals known as the Unicorn of the Sea?

Is a Narwhal's long tusk actually a tooth? Which animal is the Narwhal a relative of?  How long can Narwhals stay underwater? For free home activities sign-up at www.WhoSmarted.com See Privacy Policy at https://art19.com/privacy and California Privacy Notice at https://art19.com/privacy#do-not-sell-my-info.