Begin in Cairnes. Located in north Queensland, Cairnes is considered the diving capital of the Barrier Reef, as it is closer to the coast there than further south. So in addition to mere proximity, this means heavy competition among tour companies (read: better quality), as well as a wide range of services and prices offered.
Watch the skill level. When booking a tour, bear in mind your swimming ability and that of your fellow travelers. If you will be with children or those who are less than confident in their abilities, be sure that "Float Coats," or snorkeling jackets, will be available.
Remember the wildlife. While your chances of meeting a shark of the man-eating variety are extremely slim, you may encounter a shark of a more harmless nature, generally the timid Reef Shark, on your trip. If you or someone in your party has an absolute, unmitigated terror of sharks, check with the tour company. While no one can guarantee what kinds of marine life you will see, some companies feed the sharks in order to attract them to the boats, giving tourists more bang for their buck.
Think about your audience. Some tour companies offer a bit of Reef education before heading out. If you will be traveling with children or teenagers, if may be a good idea for them to have some good information about the Reef under their belts before being set loose upon it.
Consider motion sickness medication. The day of the trip, plan a light breakfast, and bring the ginger ale, or whatever else gets you through. The trip to the Reef will top an hour at least, longer depending on your particular destination, and the waters will be choppy. Even stronger stomachs may take a hit under such conditions.
Pack a light jacket, preferably something at least a bit waterproof. Even if the day begins hot and clear, your late afternoon/early evening trip back from the Reef is likely to be windy and cool. And if you are having the seasickness problem, you may prefer to sit out in the fresh air (and spray) during the trip rather than down below.
