The revellers had just left a floating bar and were heading to a nearby ranch for the night.
The boat operator failed to see the unlit pier in the darkness and crashed into it, according to reports.
There were 15 people onboard at the time.
They all came from the city of Franca in São Paulo State, Brazil.
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The occupants fell into the Rio Grande river between the cities of Sacramento and Rifaina on Saturday (21 Feb) night.
Several became trapped underneath the heavy hull.
Six people lost their lives, including a five-year-old child, while nine other occupants managed to survive.
They were helped to safety by a group of locals at the scene before the emergency services arrived, as reported by Need To Know.
According to the Sacramento Fire Department, only three of the 15 occupants were wearing life jackets on the speedboat.
They also said that the skipper was not licensed to operate boats.
Two of the victims have been identified as Viviane Aparecida Aredes, who turned 36 on the day she died, and her five-year-old son, Bento Aredes Ferreira.
The other victims were Marina Rodrigues Matias, 22; Juliana Fernanda de Oliveira Silva Ferreira, 40; Erica Fernanda Leal Lima, 40; and Wesley Carlos da Costa, 45, who was piloting the speedboat at the time of the accident.
It is currently unclear whether they were killed by the impact of the crash or from drowning.
The investigation into the fatal crash is ongoing.
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