I agree with both of the posts, but another thing is that the weather is alot different out on the reef than it can be in Port itself..
I would recommend any company except for quicksilver. I have been out with quicksilver, haba and poisiden. And being in the hospitality industry here in Port Douglas, I hear alot of comments from other tourists.
Generally the smaller companys are alot better, with the maximum of about 20 guests. the things I don't like with quicksilver is they pack the people on, there is no personal service and because all their snorkling is done from their personal pantoon, because of overuse the coral is dying. you also go to 3 different sites with smaller groups. slightly less time in the water, as quicksilver leaves 2 hours later than the small boats, but you see so much more.