Mariah Carey Brett Ratner on The LOVE BOAT!!! (PHOTO)

Mariah Carey and Brett Ratner have pulled a Swift/Harris ... they are full-throttle together.

This pic was taken Saturday on a yacht in St. Barts. Sources -- let's just say close to the boat -- say they're all over each other. She's totally into him and he is nuts for her.

Ratner recently said he and Mariah were just really good friends, calling rumors of romance "ridiculous." Well now that's just ridiculous, because they're in full romance mode. We're told she's over Nick and moved on.

Ratner has directed a bunch of films, including "Rush Hour" and "X Men: The Last Stand." He also directed music videos, including Mariah's "Obsessed," and "Touch My Body."

It's very age appropriate -- they're exactly one year apart. Her birthday was Saturday and his Friday.