I don't know that anyone has done a study in the Med but Robert Bednarik trashed the "vegetation mat" idea in the Pacific.
http://www.freerepublic.com/focus/f-news/1006058/posts
http://www.freerepublic.com/focus/f-news/1006058/posts
Quote:If hardy teams of H. erectus reached Flores by sea, their mode of transportation remains unknown. Some scientists suspect that small numbers of Stone Age folk accidentally drifted as far as Flores after climbing onto thick mats of vegetation that sometimes form near the Southeast Asian coast.
That speculation doesn't float, contends Bednarik. Only a craft propelled by its occupants could negotiate the treacherous straits separating one Indonesian island from the next. To back up that claim, he launched a project in 1996 to determine what Stone Age groups would have had to do, at a minimum, to reach Flores and its neighboring islands. A lot of hard work, a handful of sea excursions, and a few close calls later, he and his comrades thrust their newest and most improved bamboo raft, dubbed Nale Tasih 4, into the Lombok Strait.