John Bishop, of Compass Way in Swanwick, died on March 12 last year, an inquest heard.
The 26-year-old was last seen on February 28 and following a search by police, his body was discovered on Hackett’s Marsh.
The inquest, which concluded on August 9 at Portsmouth Coroner’s Court, heard that Mr Bishop entered the water.
However, it is not known when or why he did this.
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Marks in the mud on the saltmarsh seen by police showed that Mr Bishop made his way to the bank from the river by crawling.
But the reason why he entered the water in the first place is unknown, according to Hampshire coroner Jason Pegg.
After looking at the evidence, Mr Pegg said that it is ‘clear’ Mr Bishop made his way to the mound from the water and his body ‘was not deposited’.
“It’s unclear whether he jumped in, but it’s known he had suicidal tendencies,” Mr Pegg added.
Concerns were raised on February 28 when Mr Bishop’s carer Scott Court was unable to contact him.
Mr Court was travelling to Durham to see family when he ordered takeaway meals to Mr Bishop’s home address.
But when the meals were unable to be delivered as no-one was answering the door, Mr Court contacted the police.
A search was conducted by officers and on March 12, Harbour Master Jason Scott was contacted by the force.
In his statement, read out by Mr Pegg, he said: “It’s noteworthy that where John Bishop was found was an elevated saltmarsh.
“The level of the saltmarsh would not have been submerged at any time by the tides, between February 28 and March 12.
“His body would never have been covered by water.”
In the statement, Mr Scott went on to say how the average temperature of the water during that time was 8.7 degrees Celsius.
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The coroner gave the cause of death as hypothermia, severe ketoacidosis and immersion in water.
Concluding, he said: “John Bishop removed himself from the River Hamble by crawling onto the saltmarsh where he remained.
“Lying there in his wet clothing meant he developed hypothermia and severe ketoacidosis. How he came to be on the River Hamble can’t be ascertained.
“Therefore, I will make an open conclusion as there are a number of possibilities.
“John Bishop could have deliberately entered the water or accidentally entered the water.
“I give my condolences and best wishes to the family of John Bishop.”