The Wave Lagoon is just great for all the family. Plenty of room around the outside to lay back and relax, whilst the kids have a ball swimming in the wave pool. The waves machine runs quite regularly, with around 20 minutes on and a 10 minute break in between. The waves are great and suitable for all ages. They supply boards and other floating rings as well, free of charge. The cost is reasonable to get in, I think around $17AUS for a family, or $7AUS for an adult.
Close to shops and amenities, so you can eat close by, right near the Darwin Waterfront. We had dinner on the Waterfront after spending an afternoon swimming. Certainly a place to go if you are in Darwin..