Occupy the Marina

I'm curious about the mystery birds that occupy the marina these days. So far I've been unable to identify them. Most of the time I see them at twilight, when it's too dark to get a good look or take a good a picture. I got these pictures the other day which shows them a bit more clearly.

Cloud Nine
They sit on the raft by Cloud Nine that the ducks used to occupy. They are very large birds that almost have the shape of a hawk.

Swinging

The birds can be seen swinging on the rope that ties the boat to the dock.

Gulls just like to have fun.

Diving

I watched one bird expertly swoop into the water and come up with a fish. They act like gulls but they don't exactly look like gulls. Could they be some other kind of water bird or maybe mutant gigantic seagulls? They seem to enjoy their isolation, whereas most gulls stick together in large flocks.

Spotting the fish

 

Making the dive