Loons are similar on the water, but lack hooked bills. Most loons hold their bills level while swimming while cormorants hold theirs angled upwards. Anhinga has a long, pointed bill and a much longer tail. All adult cormorant species in the U.S. are separable by the shape and color of the gular areas. No other species has a yellow gular region bordered by a white chin patch. Double-crested Cormorants have straight orange gular areas that are dark-bordered. Immature Double-cresteds have white chests and dark bellies, the opposite of the immature Great Cormorant. At a distance, Great Cormorants appear larger, with heavier bills than Double-Crested Cormorants.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.