Initially this looks like a low-flying craft skimming over the water with a shadow. But I think, on zooming in, that it's actually just four birds and parallax.
Stabilized
Zoomed in:
You can practically see them flapping
The camera is tracking the boats, but that does no mean it's flying in the same direction as the boats. It coudl be flying left and panning right - which would give a parallax effect.
