Rob Gronkowski -- IT'S MOTORBOAT TIME ... Unleashes On Boobies

Here's NFL star Rob Gronkowski ... motorboatin' the HELL out of a pair of boobies in a South Carolina nightclub ... because of course he did.

Gronk was partying in the VIP section of Club Trio in Charleston on Sunday night -- when he dropped anchor in front of a woman holding a bottle of booze and went full "Wedding Crashers."

By the way, start to finish ... Gronk spent a full 5 seconds in between the cleavage ... while everyone around cheered and high-fived.

You motorboatin' son of a bitch ...