Justin Bieber: Caution In Iceland ... The Water Is F-f-freezing

Justin Bieber pulled a dumb and dangerous stunt when he jumped into glacial water in a remote part of Iceland for his "I'll Show You" music vid -- and people there are worried about copycat Beliebers.

Justin shot the frigid scene in the Jökulsárlón glacial lagoon ... and while it doesn't appear too painful in the video, truth is the water could be deadly ... according to Jökulsárlón ferðaþjónusta ehf (common pronunciation) -- a tour company that works those waters.

Folks at the company tell us the water temp ranges between 3 degrees and -3 degrees Celsius -- so, if the temperature doesn't kill you, an iceberg might. We're told several are floating in the lagoon, and frequently flip and break up unexpectedly.

On top of all that ... the closest town for help is an hour away.

The tour company says there are no actual rules against swimming in Jökulsárlón -- it's just stupid, so they don't expect people to do it. At least, they didn't until Justin came along.