The Blue Viper Gondola Gang was a team of three speed boat drivers in Venice. They were hired by Don Octavio to take out Carmelita Fox, but after a quick speedboat chase, Sly and Bentley took out the trio and saved Carmelita, who had no idea what happened.
Despite their name, only one of them has a blue boat. It is most likely that the blue one is the leader (probably named after the leader given the name Blue Viper Gang) and the rest are his gang members.
When the mission starts, Bentley says to Sly in the binocucom that Octavio hired the Blue Vipers to destroy Carmelita, but in the job you hunted them, they only jumped and escaped from Sly and Bentley without actually fighting them. The only thing dangerous for Sly and Bentley were the boats and docks in the canal.