COASTGUARDS are urging members of the public to report suspicious sightings immediately.
It comes after the Weymouth RNLI Inshore lifeboat searched for a possible partially sunken dinghy just off Portland Harbour on Wednesday.
The incident happened at around 4.15pm.
The rescue team found what turned out to be debris, which matched the description.
A spokesman for Portland Coastguard said: “The member of the public who reported this sighting only did so after a three-hour delay.
“HM Coastguard stresses the importance of reporting any suspicious sightings as quickly as possible.
“Time is a critical factor in any Marine or Coastal Search & Rescue situation.”
Elsewhere, concern was raised after an alert was issued from the yacht ‘Aeolus’ at around 10.30pm.
Portland coastguards discovered that the vessel was in the western Solent and the alert had been sent in error.