"It's no fluke that we're having fun… Cause were together on the water!" the supermodel captioned one photo, which showed mother and daughter smiling for the camera in halter bikinis. While Alexa posed in a white two-piece, Brinkley -- who graced three consecutive covers of Sports Illustrated's Swimsuit Issue in the '80s -- paired her navy and white striped bikini top with modest navy shorts.
In another picture, the 59-year-old Hamptons fixture looks ecstatic as a shirtless Jack shows off his catch, a huge flounder. "Jack caught a DOORMAT!" the mom of three wrote of her son with ex-husband Richard Taubman (Jack was later adopted by ex-husband Peter Cook).
The social media-savvy model also sweetly quoted lyrics from the song "Lullabye (Goodnight, My Angel)" by Alexa's father, Billy Joel, to caption a pic of their daughter resting under a towel: "Like a boat out on the ocean, I'm rocking you to sleep."
Brinkley has another daughter, Sailor, 15, with ex Cook. They split -- amid much scandal -- back in 2008.