Great Britain lost their opening fixture of London 2012, falling 13-4 to Romania in the Water Polo Arena.
The hosts face a tough task advancing from a group that includes Hungary, who have won the last three titles, and Serbia.
Captain Craig Figes scored GB's opening goal with a penalty, while Rob Parker (2) and Jack Waller also netted.
"The scoreline is a bit harsh," said Figes. "We created a lot of chances, but we didn't put those chances away."
Britain now play Serbia, who upset Hungary in a hard-fought 14-10 win in front of 5,000 fans.
"We love the challenge," said Figes. "We're playing the best teams in the world, so there's no pressure on us building towards the next game."
In the other Group B game, USA edged Montenegro 8-7.
In Group A, world champions Italy opened their campaign with a 8-5 win against Australia, while Croatia and Spain both recorded victories against Greece (8-6) and Kazakhstan (14-6) respectively.
The top four teams in each group of six go through to the quarter-finals.