The accident off Tarbert on the shores of the Loch Fyne, Argyll, happened at about 1.45am, Strathclyde Police said.
The boat hit rocks when it was about 300 yards from shore. Two people were thrown into the water and another four were trapped in the vessel's wheelhouse.
A helicopter was scrambled from Prestwick to search for survivors. The injured are being treated at the Mid Argyll Hospital. Their condition is unknown.
A Clyde Coastguard spokesman said: "We received a call from the crew of a yacht who had heard a loud crash at 1.52am.
"We think the speedboat was entering East Loch Tarbert, at the entrance to Tarbert Harbour. On further investigation, a speedboat was found on the rocks at the harbour and six people were involved."
Police and the Marine Accident Investigation Branch will investigate the incident.Reuse content