There's no joke here, but that's exactly what went down in Sardinia over the weekend ... with Tom Brady and Leonardo DiCaprio hanging out on a big boat together.

As we previously reported, the "Departed" actor has been yacht hopping for about a week following the Cannes Film Festival ... most recently ditching his shirt to hang with his guy friends out on the waters near the Italian island.

As it turns out, the NFL legend was also part of the fun at sea ... with TB12 and Leo appearing in a big group photo posted on Miami hospitality entrepreneur David Grutman's Instagram stories.

The occasion appeared to be a birthday bash for their friend, Bert Hedaya ... and judging by the picture, it didn't appear to be awkward at all for the two -- granted, they were about 25 feet away from each other.

Remember, Leo dated Gisele for more than 5 years in the early 2000s before she tied the knot with Brady ... a marriage that lasted over a decade before ending in divorce this past October.

TB12 and Leo are both seemingly keeping their options open for now ... and apparently, they're enjoying the hell outta their free time.

Grutman also posted a solo shot alongside Brady ... with the quarterback joking he was "ready for my hospitality internship."