I also recommend Lanta Divers, I did my DM with them and have nothing but good things to say. They are a very well run shop, professional and customer service oriented. However, going to Hin Daeng/Muang with Lanta Diver is a 3.5 hour journey one way as they take big dive boats from Saladaan. It would be a 6am pickup and return to the hotel at 6pm. There are really two departure points form Lanta, Saladaan the main town at the north, and the southern pier, (in high season, Pimilai Resort / Scuba shop will setup a floating pontoon pier from their beach). If it were my vacation, I would hit up one of the speed boats heading to Hin Daeng/Muang from the southern tip of Lanta, cutting the journey time to about an hour. It would be an 8am pickup and you'd be back in your hotel by 3pm. But the speedboat ride can be quite bumpy if the seasons aren't flat. Pimilai divers is the most expensive on the island I think, but they serve brie and wine, an overall first class experience.