If your itinerary is Similans/Surins, you will go and dive Elephant Head and Richelue rock. (Try and do multiple dives at each) I'm not familiar with that liveaboard as we only checked out those from Phi Phi (which leave from the same pier in Phuket). We had a great itinerary, this was the boat's third trip and they are trying to promote it (www.white-bluedive.com) 15 passengers (three were divemasters, two locally) and 4 divemasters/instructors.
There are some great dives, I would highly recomend multiple dives at both Elephant Head and Richelue rock (Richelue can get crowded with large liveaboards, so try and get an early dive in, we did a 7am 10am and a 2 there, the 7am was the best because we were the first in).
I would highly recommend trying to get out to Hin Daueng/Hin Maueng for a day, possibly two (there are some two day/two night trips where you get two dives at Hin Daueng, two at Hing Meung and 4 at this 5 island group on the way back to Phi Phi). Everyone rates them as two of the best sites in Thai waters. Check out the Paul Lee book (he saw three Manta's at Richelue Rock on X-mas day, 1996).
If you have time before or after your liveaboard, Phi Phi Island Village is a slice of heaven. A remote little village far from the crowds of Phi Phi or Phuket. They have a gold palm dive shop on site, a little expensive, but worth it, in my opinion. If you do the standard 3 dive trip to Anenome Reef, Shark Point and King Cruiser, book that out of town (Ton Sai Bay) or Phuket, it is considerably cheaper (3200 baht). The two most respected dive shops on that island are Mosquito (they have a web site, and a liveaboard), and White and Blue (we went on their liveaboard, very nice).
Visit
http://www.travelorg.com/thailand. The moderator of that site has travelled Thailand extensively. He highly recommends PP Island Village, and after a week in the Chiang Mei area, we realized that his advice was right on the money (you never know if people have a fiscal interest and promote second rate things). This guy, Mark, is true to his word; and PP Island Village was the nicest place we stayed (I have digital photos if you would like).. I have to run, I'll try and check this board this weekend when I have my dive log with me. There are a couple of off the beaten path dives in the Similans that I would highly recomend.
Matt