Mariah Carey -- My Billionaire BF is My Hero (VIDEO)

Mariah Carey and the billionaire she hooked up with on a boat may be more than a summer fling, because Mariah told several thousand people last night he's her hero.

Carey gave a shout-out to James Packer during her Vegas show Wednesday, telling the crowd she has "a new heart" and credits Jimbo.

Mariah then dedicates "Hero" to her BF.

We're told Mariah and Jim reunited this week after spending 2 weeks apart ... Jim was traveling for business. He got back Tuesday and took Mariah to dinner at Andiron Steak and Sea in Vegas ... and also had 1,000 roses delivered to her house.

Big day for 1-800-ROSES.