Humpback whales doing what they do, off Moss Landing, California. They kayakers were fine. From the YouTube post:
On our 08:00 am Sanctuary Cruises whale tour, just outside the harbor in Moss Landing, two kayakers on a tandam kayak were almost crushed to death by a massive, near full-size humpback whale. We stopped to see a large aggregation of humpbacks feeding and carrying on with random acts of hijinks. There were also a lot of kayakers right in the middle of it all. Humpbacks were coming up next to and in the middle of many kayakers. It was amusing. It's all fun and games until someone gets jumped on. The next thing we knew, this thing launched right on top of these two kayakers. That was heavy. The video was shot by Sanctuary Cruises passenger Larry Plants.
More: "Humpback whale breaches on top of kayakers in Moss Landing" (KSBW)