I bought a 5"10 surfboard snapped in half for six pounds last year, glassed it together and glassed a rope through it so its attached to a tow rope from underneath and has a handle on top to hold on to (cos its for my kids, adults can just hold a ski rope). My 5 year old and 8 year old have been "waterskiing" on it all summer behind my hobie 16 and loved it. With the kids it barely slows the boat down especially once its full on planing. I've used small windsurfers and surfboards behind speedboats down to about 15 hp I think, and I think to pull an adult on a hobie the ideal would be a suitable sized surfboard, i.e. between 6 and 8 foot, if you start lying down and then step up to your feet like a surfer then it doesnt need so much force to get started and I reckon 15 knots d be enough if not less. remember if you really turn hard on the board it slows the boat a lot so you need a fair bit of power
The more floatation you have the easier the take off. Definately recommend it 
the kids absolutely love it 