The issue is that neither look enormously like the UU, the Polynesian embarked raft looks more like it, admittedly, but then again, it is quite recognizable, and is used ingame. The cargo ship is pretty unrecognizable, but doesn't really look as much like the ships as the Polynesian raft.
Hmm, quite a conundrum >.<
On another note, is anyone else doing or planning a Cuman civ in the near future? If not then I'm taking it, one of the few Khanates I'm interested in doing it. And then I really must stop going through wikipedia late at night or I'll decide to do too many civs o.o