Tom Brady is on dad time right now -- taking his kids out to sea ... just a few days after their mom, Gisele, flew 'em out to Central America with that jiu-jitsu guy she's always with.

The retired NFL quarterback posted a bunch of photos of him and his little ones Sunday, where they're on a boat and diving into the ocean with snorkeling gear on. He captioned his post, I did Michigan and Massachusetts winters for a long time. Let me have this."

The pics he threw up are super cute ... but he definitely added one thirst trap, which features him facing the camera and completely shirtless. Yeah, the dude's ripped here.

No sign of his eldest son, Jack ... but we can see his other 2 kids -- whom he shares with Gisele -- front and center and ready to dive. There was another little girl in the mix too, but it's unclear who she she is. In any case, if you peep his IG, looks like lobster's for dinner.

Now, what's interesting about this excursion is the fact that Gisele took these same children on a trip just earlier this week. They were photographed down in Costa Rica enjoying a vacation ... with none other than Joaquim Valente tagging along as well.

Gisele and Joaquin have been on a lot of getaways together this year ... and at times, her and Tom's children have joined them. It seems like that's becoming a regular thing now.

Despite the obvious appearance of Gisele and Joaquim being a couple ... she's denied any such thing, saying he's simply a close friend of the family who coaches them all up.

Hard to say if anyone's really buying that ... frankly, nobody tends to jet set with their jiu-jitsu instructor for intimate family outings like this, month in and month out. Whatever though ... right now, it's Tom's time with the kiddos -- and he's making the most out of it.