The guy who runs yourkrabicom tends to keep things up to date - is suggesting there are afternoon boats in low season, the last of which leaves 1500.
http://www.yourkrabi.com/getting-to-krabi.php
This will still need a pretty early start from Hat Yai - it's a fair distance. So show up at the bus station early - there are fairly frequent departures.
Krabi bus station is 5km out of town and maybe 7km from the pier. Plenty of songthaews go down to town where a travel agency will be able to sell you a ticket with free shuttle to the pier. Or you can catch a taxi/motorcycle taxi direct to the pier. Even better, the lady who runs a small travel agency adjacent the bus station often meets farangs off the buses - she can sell a PP ferry ticket with free shuttle to the pier. When I've used her she's been very reliable.
If you go to the pier independently and are pressed for time you can avoid the ticket office and pay on the ferry.
Note too that Apex Guesthouse in Hat Yai, a lot of other accomm places and the small travel agencies should have minibus/ferry tickets thru to Phi Phi. No need to worry about co-ordinating times here and you usually get picked up at your accommodation.
Got a fair bit of Phi Phi info and pix HERE might help your planning.
