The Dart Harbour and Navigation Authority has issued a safety warning after a collision between a rigid inflatable boat (RIB) and a commercial vessel on the River Dart narrowly avoided tragedy.

The incident happened on a sunny evening when the small RIB, carrying local people familiar with the river, collided with a commercial vessel.

According to the authority, the skipper of the RIB was sitting on the inflatable tube of the boat, giving him a sideways view of the shore but creating a blind spot towards the river. Neither vessel saw the other before the collision.

The bow of the commercial vessel struck the RIB's A-frame, flipping the smaller craft upside down and throwing those on board into the water.

None of the occupants were wearing lifejackets and, although the RIB was fitted with a kill cord, it was not in use. As a result, the outboard engine continued running after the collision, leaving its spinning propeller posing a further danger to those in the water.

Fortunately, none of those on board suffered serious injuries and were able to swim ashore. However, the harbour authority said the outcome could easily have been far more serious.

"In slightly different circumstances there could easily have been three deaths," it said.

Following the incident, the operator of the commercial vessel has also improved its lookout procedures.

The Dart Harbour and Navigation Authority is urging all river users to keep a proper visual lookout, plan their passage, wear a lifejacket and always use a kill cord.

Formed in 1975, the authority oversees a river where around 95 per cent of activity is now leisure-based, with commercial traffic accounting for the remaining five per cent.