The QB hit the beach in Miami for a little fun in the sun with his fiancee, Bre Tiesi -- who turned heads in a tiny electric green bikini ... and it was gooood.

Manziel has been training his face off for his football comeback -- recently doing sand drills with his former Cleveland Browns teammate, Joe Haden.

Of course, Bre looks amazing ... but Johnny looks great, too -- putting back some of the muscle he had lost when he was partying his way out of the NFL.