The cormorant and shag are both big, and black. Bigger than your common duck. (mallard/aylesbury)

The grebe is light brown, with paler patches on its face -- and, of course, the crest. Smaller than a duck.

The greebe swims above the surface, but when it dives it often surfaces surprisingly far away.

I've never seen a cormorant or shag on the surface. I always see them in flight, or sat on marker posts or fence posts in the rain.