A Double-Crested Cormorant (Phalacrocoracidae: Phalacrocorax auritus) quickly swam by a few times while we were snorkeling at Three Sisters Springs.

While in a different family (Anhingidae), Anhinga appear similar and are behaviorally similar to cormorants, and the easiest means to telling the two apart, in my experience, is to look for the more robust, curved bill in the cormorant.