"I am really pleased with the race. It was a good start to the season and a really tight finish," Radford said.
Webby, from Taranaki, swam with the second bunch as Olympic silver medallist and Pan Pacific champion Haley Anderson, US, claimed the honours in 1:48.01.
The Kiwi swam 1:48.47 and was also pleased with her first outing in top company.
National champion Penny Hayes was 53rd in her first international open water race, with fellow Waikato-based athlete Troy Balvert 52nd in the men's race.
Coach Sue Southgate said it was a solid start for the New Zealanders in outstanding company.
"Both Charlotte and Kane showed they are in the top tier of world open water swimmers," Southgate said.
"The fields were the largest outside of world championships, with most of the world's best here, including the 2012 Olympic medallists and 2013 world championship medallists."
The next two rounds of the series are in Balatonfured, in Hungary, on June 20 and in Setubal, in Portugal, on June 27.