Fish live in water. A person who is in a strange or uncomfortable environment may feel like a fish out of water.
A. People may get their feet into the water slowly when they start to learn swimming. "Get your feet wet" means trying to do something for the first time.
B. In many ball games, the most important person is the one who carries the ball. Such a person is the leader and he makes sure that a job gets done.
C. Birds mainly eat worms. A bird will go hungry if it cannot catch worms. The bird that gets up early is sure to get some food, so the early bird catches the worm.
D. Our feet become hot when you run. But when you are made to sit in one place, your heels stay cool. When someone keeps you waiting, you can say he is cooling your heels.
E. Some people seem to know what's going on behind them, as if they had eyes in the back of their heads.
"Eyes in the back of your head" refers to the ability to notice everything happening around you.