Two-thirds of services were suspended last Friday as CalMac staff sought reassurances over the future of the Clyde and Hebrides operations.
A tendering process is under way that could see routes change hands from the publicly-owned company to private firm Serco.
The RMT union said a further day of action will be held on Friday July 10, when workers will be asked not to book on to shifts.
Members working for Argyll Ferries will strike the same day as they too seek assurances about job security, conditions and pensions, the shipping union said.
CalMac’s contract to run the Clyde and Hebrides ferry services comes to an end next year and the Scottish Government has put the contract out to tender, in compliance with EU law.
RMT general secretary Mick Cash said: “Despite the most strenuous efforts to negotiate a settlement on CalMac, we are still not getting the clear assurances that we seek that will defend the safety-critical jobs and working conditions that go hand in hand with delivering safe and efficient lifeline ferry services to the communities that we serve.
“It is a cold, hard fact that privatisation is all about a race to the bottom in order to maximise profits and our members will not sit back while they are caught in the crossfire of the tendering process.
“Our members on Argyll Ferries now find themselves staring down the barrel of exactly those same kind of attacks on jobs and working conditions, and have delivered a massive and rock-solid mandate for action.”
The RMT is calling on Argyll Ferries to insert a term into its employment contract stating that there will be no compulsory redundancies and no changes to staffing levels or conditions of service without reaching agreement with the union.