For any sailing enthusiast, Spencer Gulf offers the best of southern waters. Sheltered somewhat from the severity of the Southern Ocean, but still a large body of water where you definitely lose sight of the coasts on either side between Wallaroo and Cowell and anywhere south of this (and a little north, too, short of the line Port Pirie to Whyalla).
There is enough water to get a very real sense of "being at sea" - which you are, and you'd better not forget it. There is plenty of water to get into trouble if you are careless or forget to watch the weather which can change rapidly.
Second Chance encounters a bit of rough weather outside of Franklin Harbour, Skippered by Neville Hank, crewed by Marg Pope.
There are safe anchorages you can find before nightfall as you cruise the Gulf. Try Wood's Point, Port Davis, and Chinaman Creek on the east coast of Spencer Gulf, and Blanche Harbour on the west just out of Port Augusta, and the false channel at Cowell. There are free moorings at Cowell, Port Broughton, and Port Victoria. For a much larger list, more detail, and a useful discussion on where to anchor up for free check out John and Inara's recommendations.
Navigating the long and winding channel at Port Broughton
The waters teem with Squid (in the cooler parts or cooler months especially), and Snapper in season, Whiting (King George and Silvers), Garfish (delicate and delectible), Snook to hook if you trail a paravane, and big meaty Blue Swimmer Crabs if you want to anchor for a while and throw a couple of crab-nets in. Tommies show up in season in good numbers. There are others but they may be harder to catch, such as Flathead. I've caught a Sea Cucumber, but I don't recommend trying to find meat on them despite what Youtube tells you.
Happy Sailors at sea, Spencer Gulf, outside Port Pirie, South Australia
There are plenty of fishies you don't want that you are likely to hook as well as ones you do; but it isn't their fault they're not delicious or even edible. You can expect to hook plenty of Puffers and an occasional Dog Shark. In season you will catch "Trumpeters", that are also known by a more unfortunate name that I won't print here, in annoying numbers, stealing your bait and wasting your time and causing you to stab your thumb getting them off your hook.