The woman, who was 25, was pulled from the water and pronounced dead at the scene on Friday evening.
A spokesperson for Police Scotland said: “Around 5.15pm on Friday, April 23, police were called to Helensburgh Pier on Colquhoun Street where it was reported a 25-year-old woman was in the water.
"A short time later, her body was recovered by the RNLI. Her family have been made aware and are being supported by officers.
"A 28-year-old man has been arrested in connection with this investigation and enquiries are continuing to establish the full circumstances surrounding her death.
"A report will be submitted to the Procurator Fiscal."