Anon, of course you must be aware that the Mishna says that it is prohibited to daven in a tree. This is because of the difficulty in having proper Kavana while in a precarious position.
Of course, even in those cases that the Mishna does allow one to daven while traveling, (while riding a donkey, wagon or raft), it prefers, as does Joseph, that if possible, one should dismount for davening (this will be somewhat difficult in the case of the raft…).
As feivel points out, it should be obvious that the driver is precluded from davening by his responsibility not to endanger his own life or that of others.