we just returned from thailand, six nights of which was spent on railay. we didn't do the day trip to phi phi - been there before - but i can tell you every morning there were large groups of people getting onto powerful looking speedboats which took off to phi phi by 9am. there are quite a few tour booking agents along walking street - the main walkway between west and east railay - and i should think you won't have any problems booking this tour once you arrive.
of course with only two nights on railay, your one and only full day will be spent in a boat and on a tour. not saying there's anything wrong with that, except to say you might miss out on railay itself.
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