Dustin Johnson's baby mama just touched down in the Caribbean with the superstar golfer and some friends ... and it didn't take long for her to bring the birthday cakes to the ocean.

Paulina rocked an itty-bitty swimsuit and took an amazing photo ... and even Johnson himself had to gawk at the view, saying, "So beautiful!"

It's bday No. 31 for Paulina this year ... but she's partying like it's 21 all over again -- the Great One's daughter was seen tearing up a local nightclub Wednesday night!!

In fact, check out video of her getting some epic bottle service ... there's fire, Titanic music, a fake ship and booze -- it's amazing.

Of course, Paulina's been known to celebrate and party like a rockstar for big occasions ... remember when she smashed her "Bring It On!" cheerleader outfit for Halloween??