Could be tow cable, but it also resembles anchor line.
True. Here's a blurb from the Jerusalem Post that has some more detail. The ship was towed from the zone. Could have dropped anchor once it got out of harm's way.
[snip] "The Sa'ar 5-class missile ship returned to the Ashdod port and was said to have sustained damage to its rear hull near the helicopter pad. After being towed from the site of the attack, the ship had its sailing capability restored and made its way to the port under its own power. The navy is investigating the incident."
[snip]
"The C-802 missiles, a senior Military Intelligence officer said Saturday, were made in China but upgraded by Iran, which had made improvements to the radar-guided system and delivered it to Hizbullah. Senior naval officers admitted Saturday night that they were taken completely by surprise by the missile attack, claiming that they did not know that Hizbullah possessed such advanced capabilities. The missile has a 100-kilometer range."
[snip]
"Officials confirmed that the anti-missile detection systems were not operating at the time of the attack since the navy was apparently unaware that Hizbullah had such capabilities. Hizbullah also has Iranian-made drones, which officials warned could serve as more accurate weapons than missiles when rigged with explosives." [Endquote]