Stay put or find another way across

I've done a lot of fast water crossings, tethering, and floatings... Most of it in less than 4' of water and even at that depth it's enough to pull you away with ease, at 15' deep and 50' across you need a boat or a tight line going across and good upper body strength.

In as little as 2' fast moving water and a tether to another person becomes useless, once your footing (or grip) is lost you will pull the tethered person with you as you float down stream.

Being tethered to anything solid in fast moving water by yourself is extremely dangerous. You slip, and now you can't get your footing but your connected and now all the water is going around you as your body is held against rapid water movement... think of drowning in seconds. IMHO tethering requires 2 people minimum, ideally a higher elevation to tether too, thick rope, a knife with each person involved.

Tiny cord (550) is TERRIBLE for a tether. You are reliant upon it being hooked up / tied to you and you can not `pull it` to reposition safely. Thicker is not only better for strength but for gripping too.

A few years ago I was in some rapids in a concrete mixing container (large black tub about size of pickup bed) just to see how it handled the rapids, fast moving water, and just fun About half way through it was thrown about so much, and had taken on so much water it sank (I had a buoy tied to it on a long rope for retrieval). I had a life vest on and floated out of the fast water hanging on to the buoy.

15' deep.
50' across.
Fast water.

W/out a proper fast moving water craft or experience crossing water even a kayak, canoe, etc can be deadly.