The engaged couple headed down to Cabo San Lucas with 49ers tight end George Kittle, fullback Kyle Juszczyk and their wives Claire and Kristin ... where they soaked up the sun in some pretty revealing bathing suits.

Check out the pics ... the whole crew's milling about on the sand before hopping in the clear blue ocean for a dip -- lots of pecs and legs for all to see.

Christian and Olivia seem pretty deep in convo in the water too -- maybe talking about wedding planning ... after all, they've been engaged for almost a year, so ya gotta imagine the wedding's comin' up!

On top of the Niners players and their significant others, Peyton Manning was also down in the same area ... though as far as we can tell he kept his shirt on. He chatted up GK, KJ and CMC at a bar near the beach.

All of this messing around in the water comes just a week after the devastating 25-22 loss to the Chiefs in the NFL's biggest game of the year ... the Niners' second loss in a Super Bowl in the last five years.

The whole trip seems to be one big healing retreat for the gang ... with Claire, Olivia and Kristin even posting a video to OC's TikTok about how much the L was affecting all of them -- though they seem in decent enough spirits.