Summarize:

Regulation water polo players are not allowed to push off of the wall or sides of the pool. You must tread water the entire time. This is incredibly exhausting, but good players use a rounded, more efficient motion to stay afloat:  Bend your knees roughly 90-degrees as if you were sitting. Keep your legs slightly wider than shoulder width apart. Rotate your right leg counterclockwise. Rotate your left leg clockwise. Time your legs so that they alternate fluidly. They are called the center forward, and sit about 2 meters (7 ft) outside of the goal to cause confusion and take close-range shots. When you get the ball as a center your goal is to quickly turn and fire a shot off.  Centers take the bulk of physical play, and must have the best endurance and strength. Centers are also called "Hole Sets" by some coaches. You should have a line of three players in front of the goal on offense -- two wings on the sides and one center forward in the middle. A wing's goal is to take long range shots, as well as pass back and forth with the center to make space for a shot.  Wings need to have quick, accurate, and long-range shots. Remember that you cannot use the walls to push off. You should be a few meters in from the wall as a wing. Flats are great passers and long-range shooter. They swim at the goal to draw defenders, which hopefully opens up space for the center forward to get free. They must race back on defense when the ball is turned offer. Flats tend to be very quick, very mobile swimmers. This is your "Point" player. They are the first ones back on defense, and the main passers of the team, seeing and setting the entire offense. They rarely shoot, instead looking for the best, most dangerous passes. The movement of your perimeter players (everyone but the center forward) is what opens up shots and breaks down good defenses. This can include swapping positions, crashing towards the goal, or rotating around the pool. The center forward can move, but they are generally needed to keep the essential space in front of the goal open in case of deflected shots or sudden shooting opportunities. You simply slide your formation down the pool on defense. The point guards the opposing center forward, and the center forward guards the point. Your wings will cover their flats and your flats cover their wings. In general, you will play either man to man or zone defense:   Man to Man Defense: Each player follows their match around the pool, no matter where they go.  Zone Defense: Each player stays in their position, trading defensive marks as the other team changes positions. If the opposing team has a very good center forward, this position is sometimes double-teamed, leaving the offenses point player alone.
Use the "eggbeater" kick to stay afloat while conserving energy. Hold one player up in front of the goal to direct your attack. Place two players at the 2-meter line, on either side of the goal, as Wings. Place your two "Flats" on the 5-meter line, between the wings and center. Keep one player behind everyone, near the half line and in the center of the pool. Keep your wings, point, and flats moving to confuse the defense. Flip the offensive formation on defense.