Bit hard for me to help as I haven't fished there for ages but suggest fish light as in 6-8lb.  best bet re fish would be yellowfin whiting maybe garfish or squid (off the jetty which can be very busy on warm nights).  Could try the entrance to the marina (outside along the breakwall) for bream etc .

Heaps of bream and even small mullas in the Patawalonga but I'm not sure you can fish there - certainly not at the marina end

 Go to the Spot on  Tackle shop in the main street of Mount Gambier and ask craig Philp or the Nelson pub the guys there will send you in the right direction. A lot of Mulloway and Bream get caught from the landings all along the river, Great fishing.  

 Heaps of bream under the boats and around the jetties at Glenelg use 4lb leader down to a small hook then use cheap fresh bread rolled onto the hook. Cast at pylons and boats let it sink then game on. Cheapest, cleanest fishing ever and if you do no good try casting at the pontoons in west lakes got a few good fish in there to 40cm. 

Glenelg in Adelaide or Glenelg River on the Vic border.
Summer time is for yellowfin whiting in the shallows along the metropolitan beach.
I would use a berley sinker with premade berley and buy some quality worms for bait