Question: In water polo, what defensive positions can be set up?
About the article "Water polo", section "Defense".

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Defensive positions are often the same positionally, but just switched from offense to defense.
For example, the centre forward or hole set, who directs the attack on offense, on defense is known as "hole D" (also known as set guard, hole guard, hole check, pit defense or two-meter defense), and guards the opposing team's center forward (also called the hole).
Defense can be played man-to-man or in zones, such as a 2–4 (four defenders along the goal line).
It can also be played as a combination of the two in what is known as an "M drop" defense, in which the point defender moves away ("sloughs off") his man into a zone in order to better defend the center position.
In this defense, the two wing defenders split the area furthest from the goal, allowing them a clearer lane for the counter-attack if their team recovers the ball.
Hole set is an offensive position in the game of water polo.
It can be referred to as either just the "hole" position or the "set."
Because this player is typically positioned on the two meter (2M) marker and in center of the opposing team's goal, the position can also be called the two-meter or simply 2M.
Other names for this position include center forward, due to the similarity between the corresponding basketball position, as well as the pit-man.
The defensive player guarding the hole set can be called the hole-D, where D stands for defense, two-meter defender, or 2M-D.
The hole set position is often reserved for the strongest, most aggressive, or most talented player on the water polo team due the required wrestling for position while treading water.  
Water polo is a team water sport.
The game consists of 4 quarters (or periods) in which the two teams attempt to score goals by throwing the ball into their opposition's goal, with the team that scores the most goals winning the game.
A team consists of 6 field players and one goalkeeper in the water at any one time.
In addition to this, teams may have substitute players (up to six under most governing bodies), including up to one substitute goalkeeper.
Water polo is a very violent and aggressive sport, meaning that minor fouls occur frequently and exclusion fouls (in which a player is suspended from the game for 20 seconds) are also common.
It is played in an all-deep pool (usually at least 1.8m deep) meaning that players need considerable stamina and endurance to play the game.
Players use a type of water treading called eggbeater.
Some specialist equipment is needed for water polo such as a water polo ball, a ball which floats on the water made from nylon; water polo caps and water polo goals, which are either floating goals or wall-mounted goals which attach to the side of the swimming pool.
In its simplest form, the game consists of swimming (with and without the ball, sometimes with water polo specific strokes), treading water (using a specific technique called the eggbeater kick or simply eggbeater), throwing and catching the ball and shooting using a single hand.
Unlike many other team sports, such as association football, players do not tend to be only an offensive or defensive player, but rather play as either of these positions depending on where the ball is.
The game is thought to have originated in Scotland in the late 19th century as a sort of "water rugby".
William Wilson is thought to have developed the game during a similar period.
The game thus developed with the formation of the London Water Polo League and has since expanded, becoming widely popular in various places around the world, notably mainland Europe and the United States of America. 
Canoe Polo also known as Kayak Polo is one of the competitive disciplines of canoeing, known simply as “polo” by its aficionados.
Polo combines canoeing and ball handling skills with an exciting contact team game, where tactics and positional play are as important as the speed and fitness of the individual athletes.
The game requires excellent teamwork and promotes both general canoeing skills and a range of other techniques unique to the sport.
Each team has five players on the pitch (and up to three substitutes), who compete to score in their opponents goal which is suspended two metres above the water.
The ball can be thrown by hand, or flicked with the paddle to pass between players and shoot at the goal.
Pitches can be set up in swimming pools or any stretch of flat water.
The canoes are specifically designed for Polo and are faster and lighter than typical kayaks which give them fantastic manoeuvrability.
Paddles are very lightweight and designed with both pulling power and ball control in mind.
Nose and tail boat bumpers, body protection, helmets and face-guards are all compulsory. 
Albert Serrán Polo (born 17 July 1984) is a Spanish professional footballer who plays for AD Alcorcón as a defender.
He can occupy all three defensive positions.