Serbia broke open a tight game Tuesday with five unanswered goals to take a 10-5 lead at halftime and the European champions cruised from there.
Andrija Prlainovic scored five goals and Filip Filipovic added four for Serbia, which is looking for its first Olympic gold in the sport as an independent nation.
Britain, which won the last of its four Olympic water polo titles in 1920, qualified for the competition as the host nation.
Serbia leads Group B with four points after the win. Britain is at the bottom of the group with zero.