Hi Limepunch,
Thank you for your question. Due to the nature of diving, there are no guarantees as to what you might see when you get in the water (which is half of the excitement of diving!), so it is hard to say whether you’d see anything different than if you snorkelled alone. One of the perks of diving, however, is being able to get up close and personal to marine life, as opposed to snorkelling which is more of a surface activity unless you consciously dive down. Further, your instructor is trained to look out for exciting marine life and will point them out to you during the dive, and they also know the dive sites thoroughly, including the locations of large coral formations, anemones with clown fish (Nemo) and common spots for turtles and other exciting creatures.
In regards to the second half of your question, you generally do have a more relaxed diving experience on a second introductory dive. Commonly, first-time introductory divers can be a little anxious about the experience and this can often affect their breathing- quicker, more rapid breathing which means the air tends to run low faster resulting in a shorter dive time. On a second dive, divers are usually a little more relaxed and at ease, which is reflected in more regular, deep breathing and the dive tends to be longer. For the best use of your time and experience on the Great Barrier Reef, we would highly recommend considering two introductory dives.