The Coastguard was alerted on Tuesday at around 5.40pm that a trawling boat carrying three men may have caught an old mine in its nets.
The boat returned the suspected explosive into the water and came back to shore at Gourock.
Bomb squads were expected to return to the riverbed and examine the "ordnance" today.
The incident caused major travel disruption as First Scotrail delayed or cancelled services.
Arygll Ferries suspended crossings between Gourock and Dunoon.
A spokesman from the Clyde Coastguard said: "There was a small fishing vessel that picked up what looked to be a small ordnance in his nets.
"The EOD is going to take a look at it."