This was Dempsey's first outing since finishing behind Dorian van Rijsselberghe at the London Games, and since the RS:X event was reinstated into the Olympic programme for Rio 2016 after it had originally been replaced by kiteboarding.
And the 32-year-old blew the cobwebs away in style as he opened up the defence of his Miami crown with finishes of second and first in the opening two races to take a one point lead over Brazilian Ricardo Santos, with van Rijsselberghe in third and Brit Elliot Carney in fifth.
In the women's RS:X event Bryony Shaw currently sits fourth after finishes of second and fifth, with fellow Brit Izzy Hamilton in eighth after the first two races.
World championship bronze medallist Megan Pascoe recovered from being black-flagged in her first race in the 2.4mR Paralympic class to finish runner up in the second, meaning she sits third overall.
And Alex Mills Barton leads the way for Britain's four Laser sailors as he sits sixth after two races, with Jack Wetherell 30th, Matt Reid 46th and Elliott Hanson 72nd.
- Sports & Recreation
- Ricardo Santos