pelagic trip last week. The bird is heavy overall, thicker-necked and thicker-billed than a Double-crested, but the two obvious field marks on an immature Great Cormorant are:
1. Clear white belly, dirty neck. On a Double-crested, the pattern is reversed, with a darker belly and white neck. If a Double-crested Cormorant has a white belly (they sometimes do), it's going to have a white neck too. The neck on Double-crested is always paler than the belly.
2. Just a bit of orange on the throat, and none on the bill. Double-crested Cormorants show orange throats and extensive orange on the bill, e.g. the bird below.
Below, an immature Double-crested Cormorant. Click to enlarge photos.