Hi,
As promised, this is my post-dive feedback: I have booked a daytrip through Namloo Divers (strictly because of the quality of the dialogue and the clarity of the provided information). We have physically met one day before (when I have arrived in Phuket), to pay. Next morning, the Siam Divers truck picked us from the front of the hotel. The trip to the pier takes about 2 hours). From there we got into speedboats. The speedboats go daily to the big liveaboard boats which they are operating in the Similan area, carrying the daytrip and also the liveaboard guests (so the big boats don't return to mainland). On the big boat everything was pretty well organized and we had no issue. The diving itself was nice, but no spectacular/big life. After the second dive, the daytrip and the departing liveaboard customers get back into speedboat to the shore. The remaining guests do one more dive before our arrival, and one more after our departure.
Doing it as a daytrip (as we did) is pretty hard on the schedule, but we had no other choice, as we were leaving south the same day.Food on boat was ok, but not spectacular.
During the same vacation I did 8 more dives with Lanta Divers from Koh Lanta (including Hin Daeng/Hin Mueng). The service on the boat of Lanta Divers was better in terms of food/drinks on board, and I have liked their boat a little more.
In terms of diving, I know I haven't seen enough of the Similan, but based strictly on what I have seen, I would say that my best diving was at Hin Daeng/Hin Mueng in terms of visibility and fish life in general, but the very best was at Koh Bida (where I have met blacktip sharks, leopard sharks, cuttlefish and a turtle). The rest of the sites that I have seen (Similan, Hin Daeng, Koh Haa) had better visibility than Koh Bida, but no spectacular sea life encountered. Of course, this is also luck dependant.