Unbeatable Force
Before you judge the mother bear, maybe you should know just why she decided to save her own life. There could very well have been some overwhelming currents in that lake that made it physically impossible to carry on with her babies clutching onto her. While you may think that she should have done everything in her power to save her offspring, she abandoned all hope and carried on alone. The mother bear would make it to the other side intact. Also remember, that in nature, should the cubs have made it but without their mother, they also probably would not have survived very long.
Make Or Break
As for the cubs, it was all that they could do to clutch on to their mother with all their strength without falling helplessly into the deadly water. Sadly, exhaustion would quickly overcome them, and they would soon let go. These babies were then plunged into the middle of a massive, freezing lake without any kind of swimming experience. Exhausted and frozen, it seemed like the rules of nature would take their toll on the vulnerable little bears.