Paraguayan Navy divers were to begin a search Thursday for those missing after a fishing boat sunk Wednesday in the Paraguay River.
At least three people were reported dead as a result of the capsizing.
The incident occurred Wednesday afternoon, local time, after a storm hit Carmelo Peralta, The Associated Press reported.
"There was a cold front in the area and satellite-estimated rainfall showed heavy thunderstorms in the area Wednesday afternoon," AccuWeather.com Senior Meteorologist Jason Nicholls said.
Rescue efforts may be hampered by more rain, Nicholls said.
"The front will remain stalled near Paraguay over the next two to three days, so more showers and thunderstorms are likely each of the next few days," he said.
The Dream Pantanal was on a fishing expedition with 16 Brazilian tourists when it capsized, The AP reported. It has a crew of 11 Paraguayans.