Gronk and his bikini model girlfriend have taken their hotness to sea ... 'cause the couple put on one hell of a twerk fest on a boat in Mexico ... and it's all on video!!

Rob Gronkowski and Camille Kostek are hanging out in Cabo with their equally-attractive friends, Eric Decker and Jessie James Decker, this week ... and it's been a great time for everyone (including us).

The Patriots superstar had a front row seat for Camille's private booty shakin' exhibition ... while everyone else on the boat cheered them on.

The gang took their talents to the beach for some volleyball ... and we're guessing Rob brought out the classic Gronk spike at some point.

Of course, the duo loves showing off what they got ... and even gave us a sneak peek of what was to come when they got to Mexico in matching swimsuits.

No word on when they'll head back to the States ... but we're sure the party is just getting started.

Meanwhile, the New England Patriots are still waiting for an official word on Gronk's NFL future. He's reportedly pondering retirement but wants some time to really think things through and reflect before making an official decision.