A Venetian water bus captain has provoked an outcry about security after allowing a tourist to take the wheel of the vessel as it crossed the lagoon at sunset.

News of the incident emerged on Sunday just as emergency workers were searching for a 50-year-old man who disappeared when his boat sank after colliding with another vessel near Pellestrina, an island in the south of the lagoon, in a separate incident early on Saturday.

Two other men were plucked from the water suffering from hypothermia after the collision.

Both incidents have highlighted safety concerns in Venice after a spike in the number of accidents on the waters surrounding the popular tourist destination.

In the latest controversy, a video posted on Facebook shows the female tourist, believed to be Japanese, taking the helm of the public water bus, known as a “vaporetto”, as the captain and another staff member stood beside her and smiled.

The person who posted the video called for immediate action against the captain while others used social media to voice their concern.

One man tweeted “there was no limit” to what a boat captain would do.

Gianluca Cuzzolin, manager of Actv, the privately-owned company that manages the city's navigation service, said disciplinary action had already been taken against the boat captain and other crew members, who had been removed from their positions.