I SUP mainly around the sunny coast and goldie and to be honest I've never seen a SUPer without a leash on, (although I do tend to avoid the Alley). I've seen a few short boarders without a leash over the years.
My board's only 8'7", 115 litres, but it still hurts like hell when it clocks me.

. I'd feel pretty ashamed if my board ever banged into someone else.