I just came back to Phuket from a 2 day, 7 dive trip to Phi Phi and other spots. Since you are new to ocean diving the one thing you will have to learn is how to deal with currents.
I had 140 dives before the trip, but most were in fresh water and none were in currents. It was a hell of a learning experience for me, not to mention tiring.
Make sure you tell your dive leader you have no experience with currents and ask him/her if you can buddy with them for safety reasons.
Do not let anyone you put in a family group such as a family of 3 needing a 4th. This happened to me and the 3rd member only cared about his parents and not about his buddy, me.
As others have said, I'd do the AOW on the boat, but that is your choice. Don't worry about depth so much. The deepest dive we did was 30 meters and only spent a very short period at that depth as well.
As for Nitrox, it's pretty much a waste of money unless you are the type of diver that finds yourself out of time before you are out of air. Given you are a new diver, I doubt this is the case.
Bottom line is, relax and enjoy.