Sir Elton John -- Cracked Out on a Boat!

Music legend Elton John bottomed out on a yacht in St. Tropez on Monday ... by flashing his lily-white ass to some unsuspecting peeps on a neighboring boat.

While goofing around on vacation with husband David Furnish and other pals, the Grammy winner dropped trou and exposed his 65-year-old backside to another boat ... which reportedly belongs to actor Michael Caine.

No word if any of the victims are pressing charges.

Don't let the sun go down on me ... or, in this case, the moon.