Only 34 overall points separated Leigh from Schaw after the field competed for 1000 points from each of the three events - slalom, tricks and jump.
The competition entailed three rounds of each event, with athletes eliminated if they couldn't obtain a top-three finish.
In slalom, the skiers have two passes of 15 seconds to cross the wake as many times as possible. They can cross the wake forwards or backwards and on two feet or one.
In tricks, the athlete has two passes of 15 seconds to complete as many different manoeuvres as possible. They have specific point values depending on the degree of difficulty. The waterskier also collects points for the start trick to become airborne.
In the jump, the skier travels over a small, fibreglass ramp. The longest successfully landed one counts from three attempts.
Schaw accrued 2825.97 points overall to William's 2794.41 and Ryan Groen's 2447.25.
The terror from Tikokino won the slalom event but was runner-up in the other two disciplines.
"I know how well he's [Leigh] been skiing so I knew I had to be the best across all three events because he specialises in the tricks, which he won."
Fifth-placed Mitch Groen won the jump event.
Schaw, who will train in Florida before the worlds, dedicates most of his achievements to his coach, Ron Groen, of Lake Inspiration in Otaki, where he trains with sons Ryan and Mitch.
New Zealand teams:
■ Open: Hugo Schaw, Georgia Groen, Ben Groen, Brooke Fitch, Mitch Groen, William Leigh, Bevan Kelly.
■ Senior: Fred Groen, Bevan Kelly, Rob Groen.
■ Juniors: William Leigh, Chad Scott, April Foster, Toby Kelly.
■ Independent: James Leen (open), Sarah Linton (open).