Personnally, I would fly into HKT. It is about a two hour drive to Krabi.

Rationale.

HDY requires travel through a portion of the area that has had recent problems in respect to the government. Possibly safe, but, why trust fate.

At HKT, if you can catch a ride to the highway, there are Krabi buses quite often. If you drive, it is an easy drive, but a one day trip will require a drop-off fee, unless you keep the car the entire time.

Boat is probably pretty, but it will take an hour or more to get to the boat. I do not believe that there is a "ferry" to Krabi.