James and his wife, Savannah -- along with Wade and Gabrielle Union -- have been staying on a yacht called the Just J's ... a top of the line ship built in 2011 ... and owned by billionaire businessman Jay Schottenstein.
Jay is big in the Ohio community ... so is LeBron (duh) -- so we're guessing that's how they know each other.
The boat is MASSIVE -- 150 feet long ... with 5 cabins, a sick bar, fancy dining room and a hot tub.
Sources tell us ... the ballers have been hanging out on land too -- recently gambling at the Atlantis casino. We're told the gang ran into Mike Tyson, who was celebrating his birthday, and they partied together.
Pretty cool, right?