What is a penalty foul, and should I whistle straight away?

A penalty foul is punished by a penalty throw to the opposing team. Examples:

• A goalkeeper or defender committing any foul within the 6 metre area if a goal would probably have resulted.
• Pulling over, pulling down or otherwise displacing the goal.
• Within the 6 metres, a defender blocking a pass or shot with two hands, playing the ball with a clenched fist, or taking the ball under water when tackled.
• An excluded player intentionally interfering with play.
• A player or substitute who is not entitled to play entering the field of play.
• The coach or a team official of the team not in possession requesting a timeout.
• A defender deliberately throwing the ball away before a free throw can be taken, or pushing it inside the 6 metre line after a free throw to avoid a direct shot.
• A defender impeding an attacker from behind within 6 metres while the attacker faces goal and is shooting.

You can delay your whistle and wait to see whether the attacker scores in the same action. If they do not score, you sanction the penalty. For the impeding-from-behind foul, you must delay the call until the shot is completed and must award the penalty unless the attacker scores. You may raise an arm to indicate a possible penalty.

Source: World Aquatics Competition Regulations Part Six, Article 10.