Johnny Manziel's NFL comeback is taking a backseat ... to boobies. 

The QB and his smokin' hot fiancee, Bre Tiesi, are on a pre-wedding vacation in Tulum, Mexico with 2 of Bre's insanely attractive lady pals. 

For some incredible reason, the women all decided to lose their tops for a quick dip in the ocean while Johnny was hanging out and taking in the sights. 

Just a few weeks ago, Manziel told us he didn't need a bachelor party before the wedding ... and now, we kinda get why. 

Oh, one more thing ... one of the women in the group is Jordan Ozuna (she's the middle girl in the group shot) -- who's currently connected to Tyga and previously dated Justin Bieber

Congrats ... to everyone.