Becky Morgan fell in the water at the Port Of Ramsgate on May 1 after a party. Michael Bowditch went clubbing after watching her die.
Bowditch, 21, had denied murder but admitted manslaughter.
He was given a five-and-a-half-year prison sentence at Maidstone Crown Court, the BBC reported today.
Bowditch and Morgan had been at a birthday party before going to the harbour arm in Ramsgate the court heard.
Simon Taylor for the prosecution told the hearing: "Mr Bowditch accepts that although he cannot say exactly how Becky Morgan came to fall into the sea, once she fell in he failed to take any steps to try to assist her.
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Sentencing Bowditch, Judge Jeremy Carey said: "You did not try in any way to help a drowning girl - not by throwing her a life buoy, not by going to her aid as some would have done, not by calling for help, not by contacting the rescue or emergency services.
"In other words, you left her to drown."
Bowditch had been due to stand trial this week after denying murdering the college student, but an admission of manslaughter was accepted by Judge Carey.
Bowditch told police they had been "mucking about" and he believed she ended up in the water.
The court heard Morgan had asked for help and told Bowditch she could not swim. He said he was unable to help.
The prosecution lawyer said: "He did not just walk away and leave Miss Morgan to perish but remained and watched her drown."
Bowditch was later seen dancing in Rokka Bar.
He called police much later and said: "I've seen someone die tonight."
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