A fellow surfer told the Daily Mail that "They weren't very good. They were just paddling about having a laugh."
Apparently the fun stayed PG (a.k.a. no strippers). Clarence House stressed that "no one externally came in" to join Prince William and his mates.
More Royal Wedding News:
Royal Wedding Stamp Snafu Tears Couple Apart
Prince William's Got a Case of Wedding Jitters
"Nonentity" Kate Middleton Bullied at Boarding School
Sneaky Pippa! The Royal Bachelorette Party Details
See Princess Di's Secret Second Wedding Dress
Check out Kate's Royal Ride to Her Wedding Ceremony
Royal Wedding Guests: Who's Coming? Who's Too Busy?