April Love Geary is 3 months pregnant, but you wouldn't know it ... at all.

Robin Thicke hit up the pool with his 22-year-old gf on their babymoon on the North Shore of Hawaii.

The couple's been dating for 3 years, and announced they are expecting Thursday -- the due date's March 1, which would have been Alan Thicke's 70th birthday.

It will be kid #2 for Robin ... who just struck a custody deal with his ex, Paula Patton, for their 7-year-old son, Julian.