Simon Dredge suffered life-threatening injuries in the crash on Wednesday.
He was travelling in the boat along with three other professional crew including his father Peter, a champion powerboat racer.
Powerboat company, Vector World said the boat was on its first test run when it overturned just after 08:20 BST.
A spokeswoman said: "We are encouraged to hear that Simon is stable and improving."
Peter Dredge, Simon Wood-Power, a powerboat racer and Lee Hurst, a powerboat engineer were all taken to Southampton General Hospital with non life-threatening injuries.
They have since been discharged.
Eyewitnesses described seeing the high-powered vessel come out of the water "like a rocket" before flipping over.
In a statement following the crash, Hampshire Constabulary said it appeared the powerboat "lost control" just before it collided with the Hamble Point cardinal mark.
The large metal buoy marks the entrance to the Hamble River.
A joint investigation into the crash is being carried out by Hampshire Police Marine Unit and the Marine Accident Investigation Branch.

The hosts lost both openers cheaply, as Nick Gubbins was caught off the bowling of James Tomlinson, and Sam Robson edged Tino Best to Jimmy Adams.
But Voges and Malan's 279-run stand put Middlesex firmly on top, before Malan edged Liam Dawson to slip for 147.
Voges remained unbeaten at the close on 128 alongside John Simpson (28) as Middlesex finished the day on 342-3.
Middlesex's third-wicket partnership leaves them in strong position to claim their first win in the County Championship this season, having drawn all six of their games so far.
Voges' ton, his first of the season, came off 200 balls, taking the former Australia batsman beyond 13,000 first-class runs in his career.
Malan was more aggressive in dismantling Hampshire's bowling attack, hitting 17 fours and two sixes before getting dismissed attempting to drive a Dawson delivery.
The visitors could take few positives from their performance, but they will remain hopeful of salvaging a draw from the game, with poor weather forecast for Tuesday's third day.

The family had asked that half the remains be returned to them with the other half scattered in the grounds.
Half were returned but the remainder had been wrongly used in an un-witnessed scattering earlier that day at  Weston Crematorium, Worle.
The two workers lost their jobs and Dignity, which runs the crematorium, apologised for the family's distress.
Instead of admitting their mistake, two members of staff, who have been described as "very experienced", tried to cover it up by using another person's ashes for the ceremony in front of the family.
Eventually, they admitted what they had done to their manager, and Dignity - which runs the crematorium on behalf of North Somerset Council - started an "extensive" disciplinary hearing.
In a statement Dignity said it was an "isolated incident" and would look at why "very experienced staff failed to follow our established procedures".
"Following disciplinary hearings, two employees are no longer in our employment. The family and North Somerset Council have been kept fully informed throughout."
A North Somerset Council spokesman said Dignity had kept it "fully informed during the process".
"We are satisfied that they have investigated the matter thoroughly."
A teenager is "stable and improving" in hospital after the prototype powerboat he was training in crashed and overturned in the Solent.
[NEXT_CONCEPT]
Adam Voges and Dawid Malan both hit tons as Middlesex dominated Hampshire on day one at Merchant Taylors' School.
[NEXT_CONCEPT]
The ashes of a stranger were scattered in front of a family by two cemetery workers trying to cover up a mistake.