The boat's passengers say it looks like a Mako Shark, and that looks about right. I think both Makos and Great Whites attack hard and then wait for their prey to bleed sufficiently to make them easy pickings. There are other videos from other passengers aboard the same boat here and here. (It was a whole boatload of fisherdudes, after all.)
Calling them shark "attacks" reminds me of the story, by Jacques Cousteau co-writer Susan Schiefelbein in "The Human, the Orchid, and the Octopus," of him asking her whether she'd describe herself as "attacking" an egg on her plate. They're just eating, he noted.
If you decide to watch, note that another seal darts by at one point, seemingly putting itself in some danger and contrary to the flight instinct that one might expect, even if it's a mate getting devoured. One of the boat passengers wonders aloud, "What's that other seal doing there?"
CAUTION: Graphic scenes.
If you're into the grisly, here's how BBC recorded a Great White attack on a seal.
And if it makes you feel any better, here's one where a Great White Shark is out-apexed atop the food chain: