They would have failed without Cera, but she couldn't have done it alone either. They were all part of the cause, so all of them killed Sharptooth.
He could have recovered underwater and swam out, recent research shows a T. rex didn't need arms to swim. The lake was small, so though swimming would have taken a lot effort for a T.rex, I have no doubt he would have made it to the shore. But we will never know.