Excuse us as we wash our eyes with holy water.

The former hooper posted the video to Instagram on Wednesday ... where he recorded himself swimming in a pool. But, the only issue is -- the big fella was buck nekkid and gave no warning to anyone watching the vid ... aside from a clever caption.

"Morning swim be the best," a nude Davis said. "Have a cheeky day."

As Davis makes his first move to begin swimming, his ass comes completely out of the water ... revealing his decision to go au naturel for his workout.

The video was interesting, to say the least ... a swimmer might criticize his technique -- that is, if THAT'S what they're somehow able to focus on.

Baby -- a 9-year NBA veteran -- was known for having a youthful personality and being a solid role player for the Celtics ... making 2 NBA Finals appearances in 3 years.

Glen's video caused quite the disturbance on social media ... with some users encouraging others to stay away from the NBA champ's page.

In the comment section below the vid, some of Glen's followers were left asking "why?" ... with some even vowing to unfollow the 36-year-old.