A somewhat rarely-used form of seafaring transportation, a raft is a simple craft that varies greatly depending on where it is encountered.
The rafts seen in Ultima I were simply the poor man's frigate; a single-person rowboat, they were a cheap solution for anyone needing to cross bodies of water. However, these rafts had no on-board weaponry, so waterborne adventurers had to rely on their own hand-held weapons -- preferably ranged ones -- for protection at sea. They could be purchased in most coastal cities in Sosaria.
Rafts resurfaced in Ultima VI, still having no weapons of any sort, but able to accommodate a full party's worth of people.
In Savage Empire, the tribes of Eodon made use of larger, multi-passenger rafts that needed at least four people using paddles to operate. Two such craft were known of: one northeast of the Pindiro Tribe (but has no paddles); the other is west of the Barako Tribe, near Topuru's island (and is complete with paddles).
The helpful Barako natives told us where to find Topuru's island. On the shore of the lake was a large raft -- so large, in fact, that it required four people to control, and I was glad we'd dragged Rafkin along -- and we poled out to Topuru's island.