Summarize:

The object of water polo is identical to soccer -- put the ball into the opposing team's goals to score, and the team with more goals wins. In regulation games, a goal is .9 meters (3 ft) tall and 3 meters wide, but you can use whatever you have around to make a good net.  No player should be able to touch the bottom of the pool. Don't have goals and want to play a casual game? You can turn over trashcans instead and play without a goalie. Regulation pools are 25–30 meters (82–98 ft) long and 17-20 meters wide. They should be about 2 meters (7 ft) deep. In regulation games, teams each have seven players -- six attackers and one goalie. However, depending on your level of competition and the size of the pool, the most important thing is to have an even number of players on each team.  In order to know who is on what team, players usually wear matching swim caps. Goalies typically each wear a separate color from both teams. If you have more than 14 people playing it is a good idea to have substitutes. Water polo is very tiring. If you're just playing for fun and have an odd number of players, you can make one player "always offense." They simply play for the team that currently has the ball. The game starts with a swim-off, where the two teams swim to get the ball and start the attack. In regulation games a machine or referee drops the ball in the center, but you can just as easily leave a ball in the center of the pool and say "go!" The team that gets the ball first is on offense first. Using both hands at the same time is a foul. The only exception to this is for the goalie, who can use both hands within a 5-meter area of the goal. If the keeper swims forward to join the attack, they too must only use one hand at a time. This rule is to prevent clutching or holding the ball for insanely long periods, making it impossible for the other team to win it back. Because you can't foul people to get the ball, one team could theoretically score one goal then hold onto it for the rest of the game. This is why water polo has a 30-second shot clock that starts as soon as a team gets the ball. If a team does not score or reset the clock in 30 seconds, the defensive team gets the ball for a free throw where it currently sits. The shot clock only resets when:  The defensive team gets clear control of the ball. The offense scores. There is a major foul. Water polo is an incredibly physical game, both above the water and below it, but almost all player-to-player contact is a foul. You can, however, go for the ball, knocking it out of a player's hands to regain possession. If you hit the ball out or the player uses two hands to protect it, the ball is yours.  You cannot touch or impede players without the ball. You cannot hold the ball underwater to avoid getting it taken. You cannot grab, kick, or push a player while ignoring the ball. The 2-meter line is marked by red lines on the sides of professional pools, and offensive players cannot cross this line unless they are carrying the ball. You cannot receive a pass if you're already past this mark, either -- you must have the ball as you cross it, or you can't cross it at all.  The goalie, of course, can be within the 2-meter line without the ball. For less serious games, remember that 2 meters (6.6 ft) is roughly the height of an average adult male. Stay this far from goal if playing a casual game. This is similar to soccer, but with one catch -- a missed shot can deflect off a defender and still be the defense's ball. However, if a goalkeeper deflects a ball away from goal and over the end line, the offense gets a corner throw.   Goal Throw: If the offense misses or deflects a shot off of a defender, not the goalie, the defensive teams gets a free throw anywhere behind the 2-meter line.  Corner Throw: If the goalie deflects the ball out, or a defender intentionally throws the ball behind their own goal, the offense gets a free pass from the edge of the pool at the 2-meter line. This means that the whole ball passes by both posts. It does not need to hit the back of the net in order to score. In regulation games, this scoring line is marked by an all white goal line. Once a team scores, they must return to their half of the pool before the game can start again. The goalie who was scored on can then pass from their goal to a teammate, restarting the game. The shot clock starts once this first pass is made. This is the regulation length of a game, so you can adjust it to your liking if you're just playing for fun. In general, there are 2-minute breaks between each quarter, with 10 minutes for halftime between the second and third quarters.  Younger age groups usually play 5-6 minute quarters. The clock stops after a major foul (exclusion) or a goal and only starts again when the ball is back in play.
Play in a rectangular pool with two goals on opposite ends. Divide into two even teams, each with one goalkeeper. Start the game by racing to the ball in the center of the pool. Hold, pass, and shoot the ball with only one hand at a time. Set a 30 second shot clock, like in basketball, to score by. Wrestle for the ball, but not the player, to win possession without fouling. Do not go closer than 2 meters (6.6 ft) from the goal unless you have the ball. Restart missed shots with a throw from goal or a corner throw depending on who touched it last. Score a goal when the entire ball passes completely over the goal line. Restart play after a goal only once both teams are in their own half. Play four separate 8-minute quarters, switching sides after the first two.