Peter knew that if he sank, everyone would know it. We all hate to make mistakes. No person likes to fail in front of others. Chances are that the other disciples were sniggering at Peter as he stepped out of the boat. In fact, they may have even teased Him once he got back in.
Personal failure is one thing, but it is absolutely no fun when other people get to see it.
There are two lessons to consider here. Be obedient and get out of the boat. If the water-walkers are dry when they get back in the boat, we need to congratulate them. If they are wet, we need to be waiting with towels. Perhaps the child got hurt.