Where are you staying ? I know some hotels provide boat transport. If you're not at one of those I'm guessing your hotel would still provide the best advice on how to get there at that time of year.
I stayed at Ao Nang and swam at Railay (via longboat), but I know people who have stayed there at the Rayavadee Resort (which provides speed boat transport).
I'm sure there would be no shortage of boat operators at Ao Nang willing to take you. The question would be whether they would all be good options if the weather isn't great, whether you want to be getting in and out of a long boat with luggage (possibly straight onto the sand), and whether you want to be looking for a boat only after you arrive. Hopefully others can advise.
But it looks like the ferry does stop at Railay West ...
http://www.globaltravelmate.com/asia...-princess.html