Joe Jonas and Sophie Turner Like Pina Coladas and PDA in the Pool

Joe Jonas is celebrating his birthday a little early ... and Sophie Turner's white and wet crop top is certainly making it a much happier occasion.

The middle Jo bro and his fiancee were spotted double fisting fruity drinks Monday in the pool before putting on a steamy PDA display at South Beach's Delano Hotel. Wednesday will be his 29th birthday.

The "Game of Thrones" star's high-cut, 2-piece swimsuit showed off her curves and her tats. Officially, we're told that's a Greer top she's rocking, but we think crop-T-ini (™) is a much more fun name.

The couple got engaged in October and have been going strong since ... even double dating last month with Priyanka Chopra and Joe's brother, Nick. No word when they're tying the knot, but the world could use another happy 'GoT' wedding.