The River Clyde is being searched after a tugboat capsized.
Two people are believed to have been aboard the vessel when it overturned.
Helensburgh RNLI lifeboat, several police launch boats and rescue crafts from vessels in the area went to the scene when the alarm was raised at around 3.30pm on Friday.
The boat had been escorting a cruise ship into the harbour when it got into difficulty, witnesses said.
It reportedly "flipped" before going on to sink.
The coastguard rescue helicopter and coastguard rescue teams were also sent to the scene to help with the search response.
Police have cordoned off the area near Custom House Quay in the Inverclyde town.
In a statement, the coastguard said: "HM Coastguard is currently responding to reports of a capsized vessel at Custom House Quay in Greenock, 24 February.
"Just before 3.30pm Coastguard Rescue Teams from Helensburgh and Greenock, a lifeboat from Helensburgh and the Coastguard helicopter from Prestwick were sent to assist and are searching the area.
"Police Scotland is also in attendance."