The ice floe that the Bus, Ralphie, Phoebe, and Liz are on is shown in the process of breaking apart. However, when the camera cuts back to them in subsequent shots, it shows the ice floe being in its previous state and repeating the same process again.
After Phoebe dips her right hand in blubber from the top cabinet at the back of the Bus, when the camera cuts to the outside shot of Ralphie banging on the window to shoo the Walrus off of the ice floe, Phoebe's right hand has no blubber on it. In the next shot inside the Bus, the blubber is back on Phoebe's hand.
When Ralphie, Phoebe, Liz, and the Bus are stranded on an ice chunk that's floating away, it is shown in the process of breaking apart while staying afloat. In reality, the Bus's weight is heavy enough that the ice chunk would collapse in seconds and sink the bus into the freezing cold ocean, entrapping all three aboard and they would all freeze to death and drown.
When Dorothy Ann looks into her geography atlas, she states that according to her research: the Arctic temperature can drop to -100°F. In reality, the coldest temperature of the Arctic ever recorded was -92.2°F. Only in Antarctica has the temperature ever been below -100°F - in this case, the coldest recorded there was -135.8°F by Satellites but -128.6°F by air machines.
The three walruses (after one of them is ordered to get off of the ice floe by Ralphie) are shown to be drifting along with the floating ice floe with the Bus, Ralphie, Phoebe, and Liz stranded on it; but their bodies are idling as if they were floating in a river when they're clearly in the Arctic Ocean.