Yep - it's true skating was just 'land surfing' until the late 70's then in the 80's it surpassed surfing to emerge as it's own sport with seperate disciplines (street/vert/freestyle) and a whole new dimension of board tricks, air etc etc came out. At that point surfing took notice and guys like Slater and Christian Fletcher began using essentially skate moves in their surfing. Later on so did the other water sports.
Just to clarify - when i said skating was harder than surfing i didn't mean at entry level I meant at the level where you start to get airborne.
Getting airborne is the easy part. Landing in one piece is where the skill level ramps up. Stuffing a landing in the water is a picnic compared to on a concrete bowl or half pipe. 
BTW I don't skate thses days and was never much chop but having been in and around skating (boards and roller) waterskiing (slalom, trick, early wakeboarding) surfing, kiting etc etc I reckon in difficulty
to start off it is:
surfing
windsurfing (jeesoos - here we go
)
kiting
skating and wakeboarding/waterskiing
doing up velcro shoes.
At the expert level IMHO
Skating is ahead by virtue of having to involve aspects of all former disciplines and the level that the bar is at these day.
then surfing - IMHO competition surfing has gone waaaayyy backwards and is as boring to watch as kite comps. Ratsh!t epileptic fits on water.
However freesurfing and the kahunas shown by paddling into big@rsed swell and riding 'power lines' is awesome.
I have left out windsurfing here cause i am not wise to the state of upper level poling.
Kiting, wakeboarding etc etc are far behind skating and surfing i reckon.
Velcro shoes.
Just my 2c tho.
.
Showing my age. Stale fish airs and kick flips were the biz in my day. Take me back to Bulleen bowl on my Ozi concave! 
