Bluewater Lagoon is a free outdoor pool complex beside the Pioneer River, useful because central Mackay is not really a beach town. It is more practical than poetic, but on a hot day it becomes one of the city's best ideas.
Pair it with the Bluewater Trail rather than treating it as a destination by itself. A swim, a river walk, and a stop at Artspace make a good half-day in the centre.