Kylie Jenner and Tyga -- Feel the Rush ... Dripping Wet in St. Barts (PHOTOS)

Kylie Jenner has NO FEAR when it comes to diving in with Tyga -- the couple did some extreme hand holding down in St. Barts.

We're downplaying ... but this actually looks like one of the cooler things they've done since going all PDA after her 18th birthday. Tyga and Kylie went flyboarding during the family vacay on the Caribbean island-- and it looks like they're nailing it already.