From all points north of Lihue (Wailua, Kapaa, Princeville, Hanalei)
Take Kuhio Highway, Route 56, south until you reach Rice Street, Route 51, in Lihue. Turn left on Rice Street and follow it all the way down to the ocean until you split off at Wilcox. This meets up with Waapa Road, which you should take right (west) along the waterfront. The parking for the Small Boat Harbor is there.
From all points south of Lihue (Poipu, Koloa, Kalaheo, Kekaha)
Take the Kaumualii Highway, Route 50, toward Lihue. Before you get to town, turn right onto Nawiliwili Road, Route 58, near the Kukui Groove Mall. Follow Nawiliwili Road until the highway splits left, and Nawiliwili goes right. Stay right. This will end at the Small Boat Harbor, with parking right there for you.