Charles went on a glass-bottomed boat to examine coral off Castaway Island, Fiji, on the last day of his two-week tour.
He chatted to children from the Malolo District School.
Fiji's oldest resident Bechu Prasad, 104, was born on the same day as the Queen Mother.
Crowds of villagers in Mr Prasad's village of Sabeto tried to shake hands with Charles.
People climbed a disused grandstand frame to try and see the prince arriving at Prince Charles Park stadium, Nadi.
Charles chatted with dignitaries as he watched the Fiji military perform the beating of the retreat at the stadium.
A huge crowd watched him on the last engagement of his tour of Australia, New Zealand and Fiji.