Always carry a passport.

During my four years as at the Canadian Consulate in New York, it never ceased to amaze me how many americans were resistant to obtaining and carrying a passport.

Technically it is not required for travel to Canada, Mexico, Bahamas, Bermuda, etc., but relying on alternative documentation is just inviting more difficulty than you need if problems arise.

1) You will always be examined more closely if you are not carrying a passport. Your birth certificate does not have your photo. If you are a naturalized US citizen (the only other acceptable proof of citizenship), the photo may well be out of date. So there will be more scrutiny of your driver's license (and potentially even a review to see if it is in the proper form, since Customs and Immigration officers generally keep up with some 400 types of travel documents, but not 50 types of driver's licenses).

2) You aren't likely to carry your passport on your person while at the beach--but you are likely to have your wallet, in which will be safely stowed your ID. So you lose your wallet, or it is lifted, and how do you prove who you are?