Why a passport is good insurance
I have enclosed a link for a very good article on why passports are the best identification you can have for travel - http://www.usatoday.com/experience/cruise/article/why-cruisers-should-get-passports/2833095/.
You can get by without one, as shown in the article, but you should just bite the bullet and get one - it's good for 10 years and you never know when the urge to travel will hit you.