JB was in Rehoboth Beach, DE Saturday -- where he and FLOTUS, Jill Biden, have a summer home in the North Shores neighborhood -- and both of them went down toward the shore to soak in some rays and enjoy the atmosphere ... a very sunny one, indeed.

Now, eyewitnesses tell us it was Jill who actually arrived on the beach first -- accompanied by a bunch of Secret Service agents -- and that Joe eventually followed with his own flock.

As you can see, the Bidens had chairs and umbrellas laid out ... and Jill even threw on some sunscreen for protection. When it comes to Joe -- he settled for a baseball cap and keeping his shirt on. Shoes, too, it seems ... you can never be too careful for POTUS!

When they were finally seated next to each other ... we're told Joe was busy reading a bunch of papers, although it's unclear what was on them. No PDA between the first couple or anything -- sounds like they were just content with being in each other's company.

Anyway, like we said ... Joe's scheduled to hit the skies Sunday for a 5-day trip overseas, where he'll touch down in England, Lithuania and Finland for a handful of engagements.

It's interesting he's gonna be out of town this week, seeing how the findings of the ongoing cocaine probe are expected to be released in the coming days. Remember, an undisclosed amount of the illicit substance was found in the West Wing over 4th of July weekend.