MS Queen Victoria had to turn around on Friday, September 23, and return to the dock.
Police investigated the accident upon the boat's return at ~5 pm that day.
However, they believed it was a hoax.
The Cunard liner was able to depart again later the same evening.
A police spokesperson said the vessel had returned to dock in Southampton to allow officers to carry out their inquiries, which had indicated the alert had been a hoax.
"Officers are continuing their investigations into the hoax. No arrests have been made at this time."
According to a spokesperson for ABP/Associated British Ports (Port Southampton's operator), the firm could not comment on the ongoing investigation.
For more Queen Victoria incidents and accidents see the ship's CruiseMinus page.