The easiest and most common way to get from Koh Samui to Railay Beach is by booking a ferry. Ferries are well equipped with all the necessary amenities, essential to travel, like air conditioning and bathrooms on board. If you don't want to spend any money on board, bring your own snacks and water.
There are many different bus-ferry combos to chose from when travelling from Koh Samui to Railay Beach, sure to match to every person's budget and travel itinerary. Both ferries and buses/vans are air conditioned, but possibly, only the ferries come equipped with bathrooms.