It depends. If you dock in Kahului, it is quite a ways as you must cross the island and head north past Lahaina. Most cruise lines will offer a shuttle to either Lahaina or Kaanapali Beach. If you anchor off of Lahaina and tender into the wharf in Lahaina Harbor, you will be right downtown in Lahaina at the Pioneer Inn. You can grab a taix to Kaanapali Beach (around $5) or take the free shuttle to Whalers Marketplace right in the heart of Kaanapali Beach.
Have a ball!