Burnell made his major senior international debut for Britain at the European Short Course Championships back in November where he was 14th in the 1500m freestyle and 22nd in the 400m.
And, part of a British squad competing at the first two World Cup events in Brazil and then Argentina six days later he swam to an impressive 17th in the men’s race.
Burnell clocked 2:02:54.61hours in Santos as Romain Beraud was victorious in 2:02:26.01 with fellow Brits Thomas Allen 23rd and Thomas Sunter 28th.
Daniel Fogg, who finished fifth in the 10km open water event at the London 2012 Olympics, didn't start while Rachael Williamson and Lucy Charles competed in the women's event.
Williamson just missed out on a place in the top 30 like Burnell, Allen and Sunter as she ranked 31st in 2:27:50.74 while Charles failed to finish.
The British team now move on to Viedma in Argentina for the second event of the 10km Marathon Swimming World Cup which takes place on Saturday.