Phuket and Phi Phi Ferries in the Phuket-Krabi region, as well as Koh Tao and Samui, run every day unless noted. However, if there are severe weather conditions, such as severe tropical storms, the ferries will not leave the port. This does not happen often, but please be advised that it is for your safety, the boat companies at the port will advise you of changes.
When you arrive on Phi Phi: Arrival at the pier on Phi Phi can be a little frustrating, it will most likely be hot and crowded with people pushing their way past you. Then you will be barked at to pay 20 baht for "To keep the sparkling clean of Phi Phi', then you will be bombarded by touts. If you arrive on the ferry and don't have a room yet, you may end up walking from one guesthouse or resort to another, and possibly not finding anywhere to stay at all. so better to book ahead. You can trust them, they are just trying to make a living. If you don't trust them, you will either have to walk to where you are staying or use one of the many longtail taxi's gathered around the pier, unless pre-booked, then the resort should be waiting for you at the pier.