Shemar Moore -- I'm Going Down!! Thanks a Ton, Homie (PHOTOS)

Shemar Moore engaged in some all-American guy fun -- flashing his sexy, rippling muscles on South Beach ... just long enough to get CRUSHED into the sand by another dude.

The "Criminal Minds" star kicked off his holiday early -- chilling with his cousin, who is such a MASSIVE slab o' man ... he tossed 6'1" Shemar around like a rag doll. A stupidly chiseled rag doll, but still ...