The party promised to “explore the potential to build more fixed links to island and remote communities” in its manifesto, published on Thursday.
As the crow flies, around six miles of open water separate the town of Gourock from Dunoon, which sits on the Cowal peninsula in the south of Argyll and Bute.
A narrower stretch of water, between Dunoon and Cloch Point - an area just southwest of Gourock - was previously highlighted in proposals for a 2.4 mile-long bridge.
Such a construction would dwarf the 1.7 mile-long Queensferry Crossing between Edinburgh and Fife, which cost taxpayers £1.35 billion.
Transport Scotland considered the suggestion as an alternative to the A83 Rest and be Thankful, which has faced frequent closures due to landslips.
In March, however, it rejected the plans, citing “significant technical challenges” because the bridge would cross waters used by large ships and submarines.