Cyrus and Jonas -- who broke up last year -- went jet-skiing near Tybee Island, Ga., Jennifer Beale of The Coastal Source reports.
Cyrus, 16, is in town filming the Nicholas Sparks' movie, The Last Song.
An insider told Usmagazine.com Tuesday that Jonas, 16, wasn't the reason for Cyrus' breakup from Gaston -- but admitted the Disney stars had been spending a lot of time together.
Cyrus, all of the Jonas Brothers and Selena Gomez (whom Nick dated after Cyrus) taped the video for their song "Pass It On" in Malibu Saturday.
On Sunday, Cyrus "said her goodbyes" to Gaston, a source said.
"He's got his own thing going on," Cyrus told Us at the Elizabeth Glaser Pediatric AIDS Foundation Celebrity Carnival in Westwood, Calif. that day. "But I am just continuing to work hard and definitely have the whole tour going on and my clothing line and the movie."