Such as swift rivers or rough seas. Also called needle ice, from the shape of the crystals, and when it's in the ocean, lolly ice. Here's a video about fazil in the Yosemite River, showing it in action -- quite cool, so to speak. It word is a late 19th century Americanism, taken from Quebeqois frasil, from French fraisil, coal cinders, which the crystals also resemble, ultimately from Latin fax, torch.
The sea first freezes as millions of tiny ice needles, creating a dense “soup” called frazil ice.