Incidentally, take a more careful look at your "wooden raft" photograph.
It isn't really a wooden raft at all of the type that a bronze-age culture could build. All of its strength and all of its cohesion derives from the iron chains in tension holding the logs together. So this design only works with a round shape like that, with most of the logs under the surface. Even so, if you took that raft into the open ocean it wouldn't hold together for a day. The logs would shift, a chain would burst, end of raft.
I did some more research on the Pacific log rafts, because I couldn't understand how they could even manage a coastal journey. There is a lot more iron in them than is apparent on the picture that you posted. Lengthwise chains are bound together by radial chains that eventually meet the circumference chains that you see in the picture, all the chains being pulled as tight as possible. A principal somewhat akin to pre-stressed concrete where a medium that is comparatively weak in tension (concrete) is pre-compressed with steel cable to lend it much greater strength. The strength of the craft comes from the chains being held apart by the mass of the logs. Additionally the outer logs are nailed together with steel tree-nails to prevent them from shifting. Similarly it is possible to build rafts completely of concrete, but the strength in such craft comes from the steel in the concrete.