Diana Nyad: Manziel Can Save Himself ... Just Like Michael Phelps

Johnny Manziel needs to cut the crap -- and follow Michael Phelps' lead when it comes to turning his life around ... so says swimming legend Diana Nyad.

The former "Dancing with the Stars" contestant says she's tight with Michael's mom -- and knows how hard Phelps and his family worked to stop his life from spiraling out of control following a string of bad decisions.

But Nyad says Phelps was able to turn things around ... and Manziel should rip a page from his book before it's too late.