The kelp itself does not have the ability to entangle people, but if you lean too close to the kelp, you will be entangled because of the water flow problem, the large number of kelp, and the friction between the kelp and each other resulting in intertwined problems.

If you really encounter this kind of thing you must be calm and see if you can untie the kelp knot, or see if there is a sharp tool at hand to cut the kelp. If you pinch it not only will it consume oxygen, but there is a chance that the kelp will get tangled tighter and tighter (increased friction).