The girl fell about 11am on Sunday aboard the $900 million Disney Dream as the ship was sailing back to Fort Lauderdale from the Bahamas.

All passengers were alerted to the drama unfolding in the waves below as the emergency code 'Mr M.O.B.' - man overboard was announced over the intercom.

Onlookers rushed to the railings, gasping as they realized a child had fallen overboard from the fourth deck of the huge vessel, plunging into the choppy waters.  

In a split-second decision, the girl's father leapt in after, determined to keep her afloat and save his daughter's life. 

Within minutes, Disney crew members also sprang into action lowering a bright yellow rescue vessel into the sea.

'We are on the Disney Dream at sea today and just witnessed a first on a cruise,' passenger Dewayne Smith wrote in a Facebook post, documenting the chaos with photos and video. 

'A little girl fell overboard and her dad jumped overboard to try to save her. Man overboard calls went up immediately. 

'The Disney crew went into action asap. Lots of praying folks on this ship! Both the little girl and dad were both successfully rescued.'

The ship, with a top speed of 25mph, was still moving at sea, adding to the drama.

It left the father and daughter some distance away from the cruise ship by the time rescuers were able to reach them.

The desperate dad managed to keep his little girl above water for almost 20 minutes as the rescue unfolded halfway between Bahamas and the Florida coastline.

'Ten-ish minutes later a lifeboat drops and they find the people within 3-5 minutes of that. The man treaded water for at least 20 minutes,' one person detailed online. 

Video from Smith showed the yellow Disney rescue vessel bobbing through the rough waters as crew members hauled the child and her father aboard, while worried guests looked on from railings.