Wow, it's 4AM but I'm about to attempt logic. Here goes nothing.
Well, the shore that was level with the water and the cliff could have been on the same side of the island. I think its possible that the log could have floated over to the shore from the cliff and they could've gotten on it from there. They probably just skipped the climbing aboard scene. I mean, it was probably a difficult process for all five of them to climb on and balance it just right, and, even though I think it would have been highly entertaining to see, I think the producers and animators just saw it as an unnecessary scene so they just jumped to the paddling.
But again, it's 4AM and I'm attempting logic, but that's my take on it.