Serbia's men's pair capsized on the Rodrigo de Freitas Lagoon as choppy waters complicated things for rowers on the first day of competition.
Milos Vasic and Nenad Benik ended up in the water as their boat overturned about halfway through their heat, which was easily won by New Zealand's Eric Murray and Hamish Bond.
After a quiet morning the wind picked up and athletes were struggling to keep their oars from bouncing off the waves on the 2-kilometer course.
Britain's Alan Campbell, who won his heat in the men's single sculls, said the conditions took him by surprise.
"He needs to spread his arms a bit more to protect us," Campbell said, nodding toward the iconic Christ the Redeemer statue towering over the rowing venue.