The only leash design for whitewater: a quick release on a waist or chest harness, attached to your PFD. The ankle leash you use in the ocean has no place on a river.
What protects you on a lake can pin you down in a river: if a line snags on rock, branch or root, even fairly modest current will hold you under — your board becomes an anchor instead of a rescue aid. The US whitewater accident database counts seven board-related deaths in five years, four of them involving a leash. The mistake is usually mundane: bringing the familiar ankle leash because it is the same board. What you want is a waist or chest harness system with a quick release, connected through your PFD's attachment point rather than straight onto your body. What gets missed: even a quick release is harder to operate under real load than it sounds — practise on land and in shallow water, not for the first time when it counts. On technical, obstacle-rich rivers, no leash at all is often the safer call. And no leash replaces self-rescue skills or a whitewater course.