Līhuʻe Airport: The only practical gateway. Poʻipū is about 25–35 minutes south by rental car; reserve one if you want to see more than the resort strip. A taxi or pre-booked shuttle is sensible for a beach-only stay, roughly $50–75 one way; do not count on a cheap, frequent airport-public-transit link.
Nawiliwili Harbor (cruise port): About 30–40 minutes to Poʻipū by pre-arranged taxi, shore transfer, or rental car. Expect a car service/taxi to cost roughly $50–75 one way. Cruise-day independent transport is limited, so arrange it before docking rather than gambling on a ride at the pier.