I had a friend attempt to film me sailing from a drone. Alas there wasn't enough wind, the gimbal thingy mucked up and all he got was an upside-down shot of the beach, and now he's moved to Melbourne.

Was about 15 knots and he said it was handling it easily. Mind you his is military spec. and cost about $3K. Thing'll fly itself home in an emergency.
I don't know about the Harvey Norman one. see below.
I don't know what they're using in this one, but it sure isn't 20 knots as they are claiming. Looks like 15 tops.
According to this page:
http://www.dji.com/product/phantom/specThe one you listed has a Maximum Flight Velocity of 10m/second.
That's 36km/h, or about 18 knots. With a reasonable margin of safety you'd be maxed out in 16 knots of wind.
Makes me wonder if quad-copters just aren't fast enough. Normal rc chopper can do easy 100km/h, but is a lot trickier to fly.
...one pilot for flying, one pilot for filming? It's ridiculous but a whole lot cheaper than a real helicopter.
(also rc choppers in general are dangerous - not for public beaches)