Just off the Townsville coast lies the Great Barrier Reef. Several ferry companies which offer cruises to the Outer Barrier Reef on a regular basis. If you are interested in scuba diving, this is also catered for with introductory weekend dives for the beginner wishing to get her/his dive ticket and more advanced dives for the experienced divers. Townsville boasts one of the best dives in Australia - the Yongola wreck.
Sailing
If you're interested in sailing, the local yachting club holds twilight sails every Wednesday evening. There is usually a few yachts that have room for extra people. The idea is to show up around 4.30pm - 5.00pm on a Wednesday and ask for a ride.