I can see the dangers of an old anchor line. Entanglement under water, and created a "trapped" situation. I'm not sure I would take a casual bank fishermans word as it being a leading cause. I have ZERO data, but I would guess the number of drawings attributed to being entangled in an underwater line is relatively low, and there are far greater dangers under the surface than an anchor line..........However, I do think it merits some discussion.
I think sinking both ends would make a larger danger than one loose and one fixed end. You would essentially be creating an underwater "hoop" if you sunk both ends. What are the options here though? You can't retrieve it, and you can't stay there forever guarding it. You can sink the other end with a weight of some sort (if you have something else onboard to sacrifice to the water), or you can leave the cut in loose and "floating". I would suppose if you're in more than ~25ft of water, getting that far under to become entangled from simply jumping off a boat (even a dive off a two story pontoon/houseboat). You would almost HAVE to be trying to get that deep. Perhaps sinking the cut end is the correct answer?
Another thought is the effect on wildlife. If you're going to lose an anchor, does a floating line, or a "hooped" line cause more danger for local wildlife? We don't have giant sea animals here in KY, but along the coast I would suspect a "hooped" line to be worse for large creatures under the surface.
I'm curious if there is a "marine standard practice" on what to do with a lost anchor and line in this situation. Should we all avoid using Nylon (or other floating material )rope so that the whole bundle naturally sinks itself?
As a side note in that video alone, there were far greater dangers than an anchor line. Saw one guy sitting on the bow rails while underway on a ski boat. Two guys going out with no shirts, and no obvious PFD's in the boat (might be under the seat, but still). Lots of "lack of PFD" use in general, and honestly I'm bad about that as well. I keep the boys in PFD's if they are on the water. Period. Then I have the throwable next to the captains chair at all times, and there are enough vests in the top of the ski locker for all adults on board. Should the fit hit the shan and we need them in a hurry, they can be handed out without a single person standing up from their seat and causing any additional chaos on top of what's already happening. That is completely glazing over the shear number of boaters in/out of that ramp at Taylorsville. We use that ramp often, and I won't let the boys walk from the truck to the boat because of all the traffic. Impatient drivers and captains create all kinds of dangers there, but that's another story.