The 32-year-old had lead the RS:X men's fleet after day one as his bid to defend his 2012 Miami crown got off to a flying start.
However Dempsey not sits third after day two following finishes of fourth and eighth, with London 2012 gold medallist Dorian Van Rijssbelberghe of Holland now in the lead after picking up two wins, and Brazilian Ricardo Santos sitting second.
But Dempsey is not about to panic across the pond, insisting it is still good practice ahead of next month's World Championships in Brazil and his ultimate goal – the Rio 2016 Olympics.
"Following the World Championships in February and March, it will be about focusing on the Olympic campaign for Rio 2016," said Dempsey, who is joined in Miami by fellow British windsurfer Elliot Carney who is currently seventh.
"I'm just trying to get back out on the water now and get as fit as possible for the World Championships.
"If you look at Rio as a light wind venue, as windsurfers we're going to have to be light, strong, fit, and technically good. It's going to be a difficult venue.
"There is a lot of current and with it being light winds it's going to be very physical. I like to mix up my training, because it can be a bit monotonous. Anything you can do to keep it interesting and different to keep you inspired."
In the women's event Brit Izzy Hamilton is seventh while compatriot Bryony Shaw is also still in the running for a spot on the podium as she sits third after day two, with Finland's Tuuli Petaja-Siren in second and defending champion Demita Vega of Mexico in first.
In the Paralympic 2.4mR class Megan Pasco's finishes of second and fourth on day two means she is now joint third with Canadian Allan Leibel.
And Alex Mills Barton continues to lead the way for Britain's Laser sailors as he sits seventh, with Jack Wetherell 27th and Matt Reid 43rd.
- Sports & Recreation