The body of Jean Hanlon, 53, was pulled from the water in Heraklion in 2009.
A post-mortem examination concluded she drowned, but information later suggested there may have been a struggle.
Her son Michael Porter has said the latest ruling by Greek authorities was "infuriating" and called for the case to be reopened.
When Ms Hanlon died, she lived in Crete for several years, working in bars and tavernas.
The mother-of-three's body was recovered from the sea off the Mediterranean island on 13 March 2009, four days after disappearing.
Although the initial post-mortem examination said Ms Hanlon drowned, information later emerged suggesting she suffered injuries consistent with a struggle.
For the last 14 years, Ms Hanlon's family have campaigned for justice, having been frustrated with the Greek police investigations.
Following a television documentary, her case was reopened in 2019, but it failed to secure a breakthrough.
The following year Ms Hanlon’s family issued a fresh appeal on the Greek TV equivalent of Crimewatch.
The third and latest investigation was reopened 2021 by police dealing with organised crime.
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