Oooh looks like you can take a ferry:
Ferries run from/to Ko Phi Phi, Ko Jum and Ko Lanta daily. Most depart in the morning. Tickets can be purchased at the dock or from tourist offices and most local guesthouses and hotels. The normal passenger ferry does not start from the center of town anymore, but from a new passenger port about 3 km outside Krabi. Free taxi transfer to the pier should be included in the price of your ticket. If your boat starts from the old piers in the center of town (Chao Fah pier or Phi Phi pier), you are most likely on a more expensive and longer tourist boat ride. Also, make sure that when you arrive at the airport the taxi driver takes you to the correct pier. Many times they'll take you to a travel agent near the old pier and sell you additional accommodations or services. Only ever buy the ticket you need. Prices will often get cheaper the closer you get to your next destination.
Ferry tickets to Ko Phi Phi are 350 baht when purchased from a travel agent.
The large displayed time-tables in the many travel agencies are just for show and the times are not to be taken literally. Ask. As of July 2007, there were two ferries a day from Krabi to Ko Phi Phi a day: 10am and 2:30pm. They add additional ferries during high-season.
The pier at Klong Jilad, for ferries to and from Ko Phi Phi has a taxi desk with posted prices: 150 baht to Krabi town, and 350 baht to the airport.