Fresh water pours out from under a bluff and across Ninini Beach during heavy rains, so the name is appropriate (Ninini means "pouring"), but it actually got its nickname, Running Waters Beach, because of an irrigation runoff that used to flow into the area.
It's a relatively small and secluded spot at the northern end of Nawiliwili Harbor where two sandy coves are separated by a lava rock formation.
It's a wonderful spot for snorkeling, swimming, and occasional bodysurfing (but can be hit quickly with dangerously high surf from sudden southern storms) and is sometimes frequented by nudists, although that's not legal in the state.
The larger of the two coves slopes slightly towards the sparkling water and is a great place to lay in the sun. The beach is about ¼ mile from Ninini Point, where the Nawiliwili Bay Lighthouse sits.
Drive along Akukini Road towards the ocean, turn right on a dirt road that follows the water, then walk from the lighthouse down trails to the beach. Or park at the golf course clubhouse near the Kauai Marriott Resort.
There's no lifeguard on duty, though, and you'll need to walk back to wherever you parked to find facilities.