Rob Gronkowski: Pool Games with Bikini Model GF, All Aboard The Gronk!

What's better than Patriots offseason workouts?

If you're Rob Gronkowski, the answer is a pool day with your smokin' hot model girlfriend!!

FYI, things have been heating up between the Patriots star and his lady friend, Camille Kostek -- an ex-Patriots cheerleader who SLAYED the Sports Illustrated swimsuit mag earlier this year.

On Tuesday, Gronk and his GF hit up Camille's grammy's house somewhere that's not Boston ... and the Pro Bowl tight end went full Optimus Prime, transforming into her HUMAN WATER SKI!

Yeah ... Camille likey.

The hot-bod couple also got in some canoodling with a cute-as-hell puppy belonging to Gronk's little bro, Glenn -- and we're starting to get the feeling this is a lot more than a spring fling.