Rob Lowe -- BLOODY SURF ACCIDENT ... Hottest Laceration Ever

Rob Lowe became the handsomest piece of shark bait in the water this weekend -- when he gashed his leg open during a surf session gone bad ... and the blood came pouring out.

As you can see in the pic, Lowe ripped open his right knee -- but before he got stitched up, the guy posed for a Instagram pic ... with the blood still streaming down his leg.

In fact, 50-year-old Lowe was so impressed with his wound, he tweeted ... "The amount of blood that can unwantedly come out of one's body is amazing."

Hot.