The "Flip or Flop" star enjoyed a Sunday Funday in Redondo Beach, CA at the Portofino Hotel & Marina ... which included some smooching with a hottie on a yacht who looks eerily similar to his ex, Christina Anstead.

It's not his former HGTV costar, though -- it's Heather Rae Young, a luxury real estate agent and one of the stars of Netflix's "Selling Sunset."

Heather works for The Oppenheim Group selling high-end pads in L.A. ... so the 2 have both the real estate biz and reality TV in common. She also does some modeling ... and was Playboy Playmate of the Month in February 2010.

We're told Tarek and Heather boated up from Newport Beach where he lives and spent the day together. It's unclear if they're officially together or still in the market ... but looks like they're having fun.

As you know ... Tarek and Christina carried through with the 8th season of their hit show despite a very public split and divorce in January 2018. They're both moving forward with solo TV projects now, and Christina already got remarried and is expecting a child with her hubby.

So, Tarek's got some catching up to do.