Sorry to be a killjoy, but - very dumb and dangerous.
Even looking past the fact that it's dark and the guy would be very hard to see by the vehicles that are simply trying to navigate their way in the storm - in a split second he could be dragged into a stormwater drain. I've seen the result as a flood rescuer (although not on a sup).
I'm sure they knew what they were doing Sailhack.
I drove down Taren Point Rd and only the middle lane was getting driven on, the water in the gutter was about 1 ft deep. There was little to no danger if they were paddling next to the gutter.
It was just a few boys having a laugh and making the most out of the conditions. No harm done