Hilary Duff is staying above water when it comes to motherhood.

Hilary's soakin' up the sun with her son, Luca, and friends over in Hawaii. She's taking it all in -- some fun on a paddle board, catching some rays and definitely some waves ... all in mommy mode.

Motherhood suits her well ... especially in a striped bathing suit, despite it being understandably PG.

But we've seen her out flaunting the goods before ... a little here and a little there for everyone to enjoy.