When they are cruising looking down you know they're following something. They will sometimes follow a school of Tuna or Dolphin or a single large fish. They tend to follow large schools of bait in hopes other sea going predators will attack and leave a few dead or injured baitfish on the surface.
When they start a rapid decent like this one they have found something to eat.
Although it spends most of its life flying over the ocean, it rarely if ever lands on the water.
Frigatebirds are the only seabirds where the male and female look strikingly different.