He told MSPs that officials from CalMac and fleet owner Caledonian Maritime Assets were out of the country “actively assessing a vessel with a view to purchase”.
Mr Dey said: “If we get this over the line, it will have a degree of positive cascade effect across the network, and additionally create the potential for us to be heading into the next summer season with a back-up vessel standing by to cover any issues which arise.”
The minister said a new ferry for Islay was being ordered along with seven replacement smaller ferries and new vessels for the Gourock and Dunoon and Kilcreggan routes.
However, Highlands and Islands Labour MSP Rhoda Grant said: “Our island communities in the west have suffered for a number of year due to lack of capacity and reliable ferries.
"This year, the addition of Covid-19 has taken the service to breaking point, with many people unable to travel.
"This is unacceptable and damages the economy and blights lives.
"The Scottish Government has so far failed to increase capacity.”
A Transport Scotland spokesperson said: “Discussions around the vessel’s suitability are ongoing.”
It communities would be consulted over its introduction if it was acquired.
News of the new ferry follows a series of breakdowns and fault among the ageing CalMac fleet which have disrupted sailings across its west coast network for several years.
Its Gourock-Dunoon route has been reduced to one vessel with reduced sailings since May for MV Argyll Flyer to have a propeller replaced and is not expected to return to service until at least September 20.
CalMac said options were being explored for chartering an additional ferry for the route.
The operator said a fault with MV Hebrides which had put the ferry out of service for a week and disrupted the Mallaig-Skye route had been “successfully repaired” and it had returned to service on Thursday.
It said a “technical issue” with the MV Larven, which had been chartered from Western Isles Cruises as a stop-gap vessel on the Mallaig-Small Isles route had also been resolved.
The latest reliability figures published by the firm showed 97.1 per cent of sailings operated on Tuesday, with 15 cancelled among the 515 scheduled.
A total of 95 per cent operated on Monday, with 21 of the 522 sailings cancelled.
CalMac said its seasonal performance to date was 95 per cent.
The Scottish Government-owned operator has been further hampered by major delays to two new large ferries, which are being built by the formerly privately-owned Ferguson Marine shipyard in Port Glasgow.
The first, Glen Sannox, should have been in service on the main Arran route by now but will not be completed until between July and September next year.
Its as-yet unnamed sister vessel, for the Skye-Harris-North Uist routes, is due to be finished between April and July 2023.
The delays led the Scottish Government to take over the yard after it collapsed into administration two years ago.
In a further setback in June, a deal to charter the spare MV Pentalina catamaran from Orkney for the main Arran route foundered at the last minute.