Justin Bieber's got reservations for 1 ... but at least he's in paradise.

The Biebs is at The Montage Hotel in Laguna Beach where he decided a morning swim oughta do the trick to start the day.

We're told Justin walked past everyone in the lobby and headed downstairs to the Pacific blue.

As you can see ... Justin goes for a quick dip. We're told he was in for 5 minutes and then just sat on the sand and took in what is clearly an amazing landscape.

He then hoofed it back to his hotel room, but not before stopping an elderly woman after spotting her dog and asking if it was a bichon.

Justin's retirement ... it does the body good.