While sis Kim rests at mom Kris Jenner's house with her new baby girl North, two-time mom Kourtney took to Malibu Sunday to spend time with friends and watch son Mason play in the waves.

Kourt relaxed on a large picnic blanket with friends -- getting particularly close to a cute guy whose look recalls Scott Disick's preppy style.

And as Mason stripped down to his fushia-colored undies and ran in the water with friends, Kourtney cuddled her little one and breastfed her daughter on the beach.

Our photographer tells us: "She was totally comfortable; it was no big deal. She wasn't embarrassed at all and none of her friends seemed surprised."

You go, KK! As it should be!

Meanwhile, Kim seems to have come around to the natural way ... reports claim the new mom loves nursing little North West -- a significant departure from her previous feelings about breastfeeding. Back in 2010, Kim was slammed for Tweeting about a woman breastfeeding in a restaurant without a coverup. Oops ...