if the lines under tension (emergency situation) the knife will cut through pretty easy. just because you dont tie a knot in a line doesnt mean it cant somehow get entangled. they are designed to cut things like throw ropes (another thing that should be in every driftboat used in whitewater), webbing, etc.
Its a super easy to carry tool that is great for mitigating some of the entrapment risk and is a common part of just about any swiftwater rescue equipment. They also have a blunted tip to reduce the risk of injuring yourself and others from stabbing during a hairy situation (note, the edges are still flesh slicers)
most of the time they are used when shit is hitting the fan, like the stern of the boat is about to go under because some accidental chain of events has occurred.
They cost 40 bucks, have a good quick release sheath, and can save your ass (or someone elses that you happen upon).
Also, if you have the skill to use it, it couldnt hurt to carry the items needed for a Z-drag (rescue grade pulleys, carabiners, rope, and webbing)