Before his official 21st birthday celebration at Windsor Castle, Prince William got wet and wild partying with friends at a “real” birthday bash, taking an after-midnight dip in a castle moat and frolicking with a pretty brunette.

The ENQUIRER has exclusive photos offering a rare glimpse of the future King’s fun-loving side — including a shot showing the Prince dancing in a tight bathing suit as a young woman wraps him in a feather boa!

The issue that goes on sale Friday also details William’s hard-drinking party at a Scottish pub, tells why Prince Charles WON’T let him have a motorcycle … and more.