The 46th POTUS hit Rehoboth Beach -- a favorite spot of the Biden clan -- for a little R&R Saturday ... and, camera crews caught him kicking his feet up underneath a beach umbrella.

Not a lot happening in this vid if we're being honest ... and, that's exactly the problem -- 'cause people online are taking Joe to task for sleeping on the job.

X users are taking to the platform to vent about how Biden's taking a vacation with just months left during his presidency ... and, many conservative commentators are asking who exactly's at the country's helm right now.

It's a line many Republicans have parroted in recent weeks after Biden announced he was dropping out of the 2024 presidential race.

Remember, after he dropped out many Congressional Republicans demanded he resign the presidency, claiming he wasn't fit for office if he wasn't fit to run ... and, they've questioned his competency for months.

BTW ... not strange to see Biden at the beach -- he's known for loving the sand and the waves. But that's infuriating these commentators online even more than calming them down.

Meanwhile, Kamala Harris and her VP pick, Tim Walz, are crisscrossing the country at the moment ... looking to draw support from the American Heartland to win the election.

Biden's public appearances are few and far between these days ... but, he may be resting up for the Democratic National Convention in two weeks time.