WHERE? If you are in north queensland, those are stinger suits to prevent jellyfish stings.
If you are anywhere BUT northen queensland, you can wear whatever you want - BUT, wetsuits can help you keep warm in cold seasons, and rash vest well, prevent rash!
I usually just go in my boardshorts and go topless or with a vest inteh summer. In the winter I wear a 3/2 or 2mm suit.
Yep - in Queesnland I think a 3/2 is more than enough for the winter, but if you live somewhere in VIC or TAS, then maybe a 4/3 or 5mm is necessary in the cold months.
Beware, that the thicker the wetsuit, the more difficult it is to paddle.