My guess: After Hafu slowed down the Tahnok by toppling the statues, the Po-Koronans used this delay to hide in the cliffs. The Tahnok smashed through the statues, destroyed the village, and moved on. But having no shelter, and fearing the return of the Tahnok, the Po-Koronans ran down the path to the beach (where Takua sailed to and from in the MNOLG). Good question about the boats, though, where would they find five boats in a hurry? Maybe they built them in Po-Koro before they left, and dragged them to the beach. They couldn't go to Onu-Koro, because it had been flooded. So the next closest village was Ga-Koro. (Or maybe Huki persuaded Onewa to go there!)
Pohatu had gone with the other Toa to find the Bohrok nest. Remember in the Le-Koro update, Takua tells Tamaru and Kongu that the Toa have all gone to find it.