Yes there were times that I would have maxed out on nitrox for the first dive, but I only ever use it on second dives. The certification is 2 dives (I think?) so I did the certification over 2 days with the certification dives as each second dive. If you have the time to do the certification over a couple of day then that would be the best solution. Hope this helps.
Also, from my experience with CA we left each morning around 8/8:30 AM and really did not get back until almost 3 PM in the afternoon. It was a long, full day with some of the best surface intervals I have ever experiences. We went cliff jumping and explored some of the caves on French Cay. I would definately do my course work at home because the time on the boat leaves little time to relax if you also have to do class work. Just my opinion.