Let's wait for the full investigation as eye witnesses state that they saw drones flying in that victim and then striking the boat that was anchored:

Fire breaks out after ‘drone strike’ on Gaza-bound flotilla carrying Greta Thunberg
Eyewitness says the explosion happened ‘right next’ to the boat’s diesel tank
The flotilla arrived in Tunisia over the weekend and was anchored about 80km from the port of Sidi Bou Said when the incident allegedly took place.
“I was sleeping when I heard this explosion,” said journalist Yusuf Omar, who is among the dozens of activists heading towards Gaza in an attempt to break the Israeli blockade. “A big explosion happened right next to our diesel tank. The neighbouring boats spotted a drone coming in. They said it flew above basically where I was sleeping, and then it went to the front and detonated,” he said in a post on Instagram.
Another eyewitness told Seen TV: “I saw a drone dropping a bomb on us... There was a fire on the front deck that we fought. It is out now. Everybody’s safe.”
The Tunisian authorities said the explosion originated inside the vessel and that reports of a suspected drone attack on the flotilla “have no basis in truth”.
Tunisia’s national guard said their investigation was ongoing, but “no drones have been detected”. “According to preliminary findings, a fire broke out in the life jackets on board a ship anchored 50 miles from the port of Sidi Bou Said, Houcem Eddine Jebabli, a spokesperson for the national guard, was quoted by AFP news agency as saying.
The Tunisian authorities suggested that the fire might have been caused by a cigarette.