The presence of large numbers of divers tells you that baitfish are abundant. Watching the behavior of the divers provides greater detail. Feeding periods for birds and fish often coincide. A frantic gathering of birds pounding the water means that bait has been driven from below. Salmon will surely push baitfish to the surface, as will black, blue and yellowtail rockfish.
Shearwaters are another bird to watch in the North Pacific.
These highly migratory birds move in groups of thousands. Krill are essential parts of the food chain. If you spot a soaring frigate, move in that direction. If the bird is high in sky, take a look at the position of the sun. Frigates position themselves to watch their targets with minimum glare.