Centre Manu Tuilagi has been formally warned by police and fined £3,000 by England rugby officials after jumping from a ferry in Auckland on Sunday.
The 20-year-old was detained by police after swimming a short distance to a pier at the ferry terminal.
"I'm really sorry. It was a silly thing to do and I apologise to everybody for any inconvenience," said Tuilagi.
The Rugby Football Union (RFU) would not comment on whether alcohol was involved in the incident.
England were knocked out of the tournament by France in the quarter-final on Saturday and left for home on Monday.
Manager Martin Johnson said: "This was an irresponsible thing to do. Manu has been disciplined internally and I have warned him about his future conduct."
The fine will be paid to the Rugby World Cup's Christchurch earthquake appeal.
Auckland Police spokeswoman Noreen Hegarty said: "A member of the England team jumped off a passenger ferry and swam a short distance to pier three at the terminal at 6.48pm last night.
"As he climbed ashore he was met by police officers before being taken to the Auckland Central police station and held for around an hour and a half.
"He was formally warned about the offence of disorderly behaviour before being released back to the team management. There is no suggestion he was pushed from the vessel."
The off-field conduct of some of the players has prompted front-page headlines throughout the tournament.
A group of players enjoyed a night out drinking in Queenstown, with permission from team manager Martin Johnson.
Captain Mike Tindall was captured on CCTV footage from the Altitude Bar in conversation with a woman. A bouncer has been charged with illegally obtaining the footage.
Tindall later issued an apology for misleading the England management over his movements that night, having initially claimed he did not go on to another bar afterwards.
Johnson also confirmed during the tournament that he had reprimanded James Haskell, Dylan Hartley and Chris Ashton after a female member of staff in their Dunedin hotel complained of being teased with lewd comments.
Tuilagi was, earlier this month, fined £4,800 by the International Rugby Board for wearing an unapproved mouthguard carrying the name of a sponsor.
The Leicester centre was also banned for five weeks back in May of this year for punching Northampton's Ashton.
Tuilagi was initially banned for 10 weeks but his suspension was reduced in light of a push from from his England team-mate.
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