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 dull yellow gular region bordered by a thin white line. Double-crested Cormorants are quite similar to Neotropic Cormorants, especially the immatures but note that the smaller Neotropic has a duller, pointed gular region, a longer tail, and a slimmer build.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.