We were taught to pretend we were pedaling a bicycle underwater while using our arms to sweep water away from us. You start with your arms shoulder height and bent at the elbows. Turn your wrists inside so that the backs of your hands are touching each other in front of your chest. Extend your arms out straight in front of you and push them away from your body.
Repeat the arm motion while pedaling your feet and you should stay afloat. Good luck!