Olivia Culpo, Christian McCaffrey

NFL superstar Christian McCaffrey and Olivia Culpo are officially at war -- don't worry, their relationship is doing just fine -- but they're now competing against each other for the best abs on the planet!!!

The Carolina Panthers RB and former Miss Universe hit the waters for a romantic meal on Monday ... eating up all the Chick-fil-A sandwiches they could fit in their shredded bodies.

OC posted a snap from the getaway on Instagram ... with the couple sharing a kiss in between bites.

"You had me at fried chicken sandwich," Culpo posted.

OC's fellow S.I. Swimsuit model friend Brooks Nader took notice of the biggest takeaway from the picture ... adding the simple caption, "Abs!"

Of course, OC and…