@Katie L
I recommend Wicked Diving out of Kao Lak for 3 day liveaboards. They alternate between the Similans and the Surins.
They cater for all levels. I found them to be safe, eco friendly and great fun. my wife and I enjoyed our trip with them. We were teh most advanced divers onboard, but never felt limited and those with less dives were never put into dives outside their comfort zone.
They do carry out AOW courses onboard too
My highest recommendation for them, is that we returned purely so that they take our two sons (then 12 and 14) on a DSD dive. As experienced divers we are very choosy with whom we entrust our children to for training.