Well looking at the sites below, I'd say that you'd need to be there by 1pm at the latest to make sure you are on the boat, but landing at 12:15pm would make that a no-no.
http://www.phi-phi.com/travel/ferry-schedule.htm
http://www.phiphi.phuket.com/schedule.htm
http://www.asiatravel.com/thailand/p.../transfer.html
Take note that they do not all leave from the same ports!!