You mean they drag a line in case the main line breaks so they can hook up a new line in rough weather without having to come alongside?
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YES,
I have seen the after lines being dragged behind barges here, also log tows. Have not paid attention in the last few years as there are FAR fewer than when I first started boating. Regardless , when I cross after a tow I stay well clear.
The ones I remember also had a good sized fender or float marking the end so it was more easily located but sometimes the float is gone, then what. I live in B.C. and I've seen them used here. The guideline used to be stay back at least 100 yards and keep your eyes peeled.
As far as the watch seeing a boat flipped, don't bet on it. Bunch of years ago a boat was flipped when they crossed the tow line. It was at night. The tug crew did not see it but according to the report they did feel a jerk so dropped power and used the searchlight before they realized what happened.
THere was a small speed boat who had tried to warn the unfortunate boat and did also help save some people. But the watch did NOT SEE and were it not for the jerk they could have carried on without being aware. Report below which I have read.
Marine Investigation Report M99W0133 - Transportation Safety Board of Canada
We have berthing tugs and they do not use wire lines for this purpose, rather synthetics, often Dyneema or Spectra. Some of those lines can float and if a piece broke away it could cause the trouble.
Even some [many] of the deep sea ships use synthetics for mooring lines rather than wire.
Now I'm going to have to go looking more since i have left Vancouver Harbour so don't see as much on a weekly or so basis.
THe line in question could indeed have been lost fishing gear depending upon the fishing rig. Some of those larger boats such as seiners use some pretty good size lines.