Ditto Carl's comments but I would also suggest going with Undersea Explorer. The boat isnt as plush as Mike Ball but we were incredibly well looked after and it has a focus on research and Biology which I found fascinating. We dived along side the marine biologists and I was even able to help with the Nautilus catch and tag project. Having the chance to hold these amazing creatures was a once in a lifetime experience. UE is certainly good value for money. We went in January which was significantly cheaper than the peak season and we had dead calm seas - glassouts on a number of occasions and bath-warm water. Virtually no rain either. As long as I live I will never forget the day with conditions so still that the captain moved the boat to within metres of the plunging reef wall and cruised slowly along it for an hour, surrounded by clear, glassy water, bright, sunny, blue skies, flying fish and not a speck of land in sight. It was magical.
No matter who you go with you'll have an incredible time. Osprey reef is stunning.
Enjoy!