At least a dozen people, including three who were taken to hospital, were involved in a seven-car pile-up after the trailer, boat and Seat Leon ended up scattered across two lanes of the RM12 near El Algar in Murcia.
Police believe two or more traffickers had stolen the semi-rigid, twin-engine craft and were heading for La Manga, where they said drugs were often smuggled to shore.
The vehicle carrying the boat fled the scene, with police trying to track down the occupants by identifying the speedboat. The Guardia Civil said that the boat, because of its characteristics, was very likely to be related to drug trafficking.
A witness told El Espanol that he was driving at 60mph when the speedboat hit the car in front of him. He said he managed to swerve to avoid the wreckage before several other vehicles sped past him and crashed.
Police have said several coastal areas in southern Spain, including La Manga, are hotspots for drug trafficking, with marijuana and cocaine shipped in large quantities into Europe from North Africa or from across the Atlantic.