Justin Bieber -- Sharks and Recreation

Now we know why things have been so quiet in Calabasas ... turns out, Justin Bieber's been on vacation with his pal Lil Za in the Bahamas... where they splashed around with an entire shiver of sharks and TMZ has the pics.

Fun Fact -- a group of sharks is called a "shiver" ... like a "murder" of crows or a "pod" of whales. Weird, right?

Anyway, Bieb and Za stripped down to their swim suits (Za appeared to be in his undies) and played in the turquoise water at the Staniel Cay Yacht Club ... where they hung out with some local Nurse sharks.

Don't worry ... there was no real danger -- while Nurse sharks can grow up to 14ft and weigh over 700 lbs, they generally don't attack humans ...

... unlike some other people we know.