What is the difference between misconduct and violent action?

Both are exclusion fouls that end the player's game, but they are punished differently.

• Misconduct: unacceptable language, aggressive play, refusing obedience to or showing disrespect for a referee or official, or behaviour against the spirit of the Rules likely to bring the game into disrepute. During the game the player is excluded for the remainder of the game, with a substitute after the earliest re-entry occurrence, and must leave the competition area. If it happens during a stoppage, interval, timeout or after a goal, the player is excluded for the remainder of the game and a substitute may enter immediately before the restart.
• Violent action: kicking, striking, or attempting to kick or strike with malicious intent an opponent or official, during play or any stoppage. During the game the player is excluded for the remainder of the game, must leave the competition area, and a penalty throw is awarded to the opposing team. The player may be substituted when four minutes of actual play have elapsed. If it happens during a stoppage, timeout, after a goal or between periods, no penalty throw is awarded, but the exclusion and four-minute substitution rule still apply.

Source: World Aquatics Competition Regulations Part Six, Articles 9.13 and 9.14.