There are now two daily long-tail boat from Khuan tungku Pier. 9.30am & noon. If you start from the train station in Trang town by van, its like 350 baht.
There are also several boats coming from Koh Lanta too.
Boat to/from Ko Kradan depart from Hat Pak Meng pier in Trang province. It takes about two hours and services are irregular (usually one daily).
Boats to Ko Kradan are less frequent that boats to Ko Muk.
You may also charter a whole boat if you can share the price with somebody else.