No boat has been found to be at fault for the deaths of two children at Bournemouth beach last month.
Sunnah Khan, 12, from High Wycombe in Buckinghamshire and 17-year-old Joseph Abbess, from Southampton died after they got into difficulty in the water at the Dorset resort on May 31.
At the time it was understood that larger than usual waves were seen in the sea at the time, with wash from a passing boat being suggested as a potential cause.
Eight other people were rescued at the scene and treated by paramedics for non-life-threatening injuries.
Bournemouth, Christchurch and Poole Council banned all boat operations pending the police investigation following the tragedy.
Today, the Marine Accident Investigation Branch (MAIB) has said it will not be launching a formal investigation into the deaths of the two youngsters.
Dorset Police had impounded the 78ft passenger vessel the Dorset Belle.
They also arrested a man aged in his 40s and who was 'on the water' at the time of suspicion of manslaughter.
A spokeswoman for the government agency said: "Having conducted a thorough preliminary assessment of the circumstances of this tragic accident the MAIB has decided that no formal investigation will be launched.
"This was a truly tragic accident but our understanding of the circumstances is that it did not involve a vessel and therefore is not something the branch would investigate."
Island Cruises Ltd, the owners of the Dorset Belle, has already vowed never to return to Bournemouth following the tragedy.
A maritime safety expert has said there was no evidence the passenger boat was to blame for the deaths and that the most obvious explanation for what happened was a riptide, which are common at Bournemouth Pier due to its location.
The expert, who wished to remain anonymous, said: "It is known that riptides are common on any open stretch of water that faces the sea that has a prevailing wind coming on to a south facing beach and shifting sands.
"Whenever you have a pier on a beach, it is inevitable there is going to be a lot of riptides.
"Because a commercial vessel was in the vicinity of the incident, it is normal that it has been impounded while they investigate.
"But the vessel would have been heavily regulated by the Maritime and Coastguard Agency with an experienced crew.
"Its design means it has no underwater suction and doesn't manoeuvre on jet propulsion so any theories about 'jet wash' causing the incident do not apply.
"The vessel has a 360 degree viewing station for its master, is highly manoeuvrable and only operates in deeper water.
"There is no evidence to prove negligence on behalf of the passenger vessel in question. The most logical explanation for what happened is a riptide.
"They are unpredictable and can be hard to spot, happening after a period of calm waters."
He said BCP Council needs to consider allowing people to swim in the sea around the pier in the future.
He added: "Why is the designated swimming area so vast and close to the pier when it is well known that currents and rips are at large there and have been for so many years?
"The council needs to look at the parameters of where the designated swimming zones are."
Inquests into the deaths of Joe Abbess and Sunnah Khan heard that they drowned after a suggestion they were caught by a sudden riptide.
