I surfed a few weeks ago on Veteran's Day (Indian River Inlet Deleware) with water temp at 58 degrees F and air temp was 72 degrees F. No boots...my feet were in pain from the cold for the first five minutes or so in the water but then they adjusted (or got numb) and I was good for about 1 1/2 hours until I started getting cold and took a break. I would say if I owned boots, I'd probably wear them anytime under 65-62 degrees just to be warm and enjoy myself.
That day I was talking about (58 degree water temp/ 72 degree air temp) my hands and head were fine..it was just the feet that were cold.