Firefighters fought what they described as "punishing conditions" to tackle a blaze on an "iconic" lightship.
Emergency services were called to the LV18 at the Quay in Harwich just after 21:00 GMT on Friday.
Essex Police said it was being treated as arson and a group of young people were reportedly seen leaving the area shortly after the fire started.
The ship, which was built in 1958, has been used for pirate radio projects in the past.
Essex Police said the incident was not believed to be linked to earlier fires at a railway station in the town.
Group manager Danny Partridge, from Essex County Fire and Rescue Service, said crews had faced "punishing conditions with extremely high temperatures in the engine room" of the ship.
He said they "worked hard to enter the engine room from the deck and prevent the fire spreading".
Crews left the scene at about 04:00 and returned to monitor temperatures throughout Saturday morning.
Tony O'Neil from the Pharos Trust, which owns the ship, said it was one of the last manned lightships in the country and "an iconic landmark".
He said he was "confronted with absolute piles of smoke" when he arrived at the scene and the hull continued to be dowsed with water even after the fire was extinguished as it was "red hot".
"There is extensive smoke damage. We will not be beaten and we will bounce back," he added.
Ivan Henderson, a Labour councillor in Harwich, said: "It is a real shame.
"It is one of the attractions that we are going to have to suffer from not having around."
BBC Essex marked 50 years of offshore pirate radio stations by broadcasting from the LV18 for seven days in April 2014. Radio Caroline was the first to launch on Easter Sunday 1964.
An Essex Police spokesperson said: "It was reported that a group of young people were seen leaving the area shortly after the fire had started and we are treating the fire as arson.
"If you have any information, CCTV, dashcam or other footage in relation to this incident, then please get in contact with us."