I have cleaned pretty grundgy docklines and anchor rodes by laying them out
on my driveway and blasting them with a pressure washer while rolling them
back and forth under foot to get at the "bottom" of the lines. No soap was
needed and the lines looked close to new. My pressure washer is electric.
I imagine that using a higher-powered gas-powered pressure washer might
involve some risk of damage to the line fibers. All the lines I have done
this on have been 3-strand nylon. I have not tried it on braided line.
Your mileage may vary.