Travel from Koh Tao to Railay beach can be done in no time (well, in 7.5 hours) by booking a bus-ferry combo. With this service option, you don't have to plan out your transfer bit by bit with the potential of missing one vehicle or the other. There's a toilet and no air conditioning on board the ferry. Although, there is air conditioning on the bus (with no bathroom on board).
When venturing out from Koh Tao to Railay Beach, the ferry ride is only part of the adventure. There are bathrooms on board the boat but keep in mind that there is very little air conditioning. Don't worry though, the seating is open air, meaning that you should get ready for the most photoshoot ready hair-swept-in-the-wind look.