it's 20 years ago, but I used to go out to the islands off Railay on the dive boats if they had space (for not much money)
many of the dives stick to the edges of the rocks sticking out of the sea, so that's where the boats are going
initially I was just using my time in between dives to climb stuff, but ended up going out specifically to just climb
might be worth asking dive companies while you're there
I did most stuff with Baby Shark Divers, but I believe Richard is no longer there