Cardi was hangin' on the beach during a vacay with Offset Saturday, when she spotted the yacht as it disappeared into the drink. Of course, she chronicled everything on video and posted it.

Cardi gives a dramatic play-by-play ... "What the f***?!!? It's sinking!!! Y'all see this?!!?"

And then the hilarious line ... "There ain't no big boat that could save it?!? She bid farewell in Cardi fashion ... "Bye-bye."

She says no one was on board the doomed vessel.

It's unclear where Cardi and Offset are ... but it looks super tropical. She posted photos of her and her hubby on a private jet en route to wherever they are.