If you book online in a beautiful resort on Railay West, do not hesitate to contact them to see if they offer boat transfer from Ao Nang. Most of them do.
The best time to come to Railay is from December to March and especially in January-February. During summer it rains oftent especially from August to the end of October, the worst period to come to Railay.
We are one week away from Chinese New Year (January 25th, 2020) and despite a lot of mail and detailed information requests from various Tourist Offices in several cities, I unfortunately did not get all the details that I would have liked...