Councillors had raised concerns about the project in Stranraer, prompting the investigation.
Auditors PWC found no evidence of staff acting "beyond delegated authority" or a failure to secure best value.
However, they listed a string of "key learning points" to help strengthen arrangements for future projects.
The report followed an internal investigation which delivered similar findings.
Local councillors had voiced concerns about changes in the specifications of the project with some claiming they had left the slipway "virtually useless".
They took their concerns to the council's monitoring officer who, in turn, contacted Audit Scotland.
PWC was subsequently appointed to carry out the best value review.
It found no rules or policies had been broken but that there were areas where project management arrangements "could have been better".
It listed a number of key learning points:
The review was based on key council documents, 18 committee reports and interviews with "key individuals" involved.
An external review of the handling of a slipway scheme in south west Scotland has found no council officers acted inappropriately or broke regulations.