One man has died and another is in a serious condition in hospital after being thrown from their speedboat.
Police have launched an investigation into the death following the incident off the Isle of Wight on Friday.
The two men, who have not been named, were dragged from the sea just off Yarmouth by the crew of a ferry.
A spokesman for Hampshire Constabulary, which has asked for witnesses to come forward, said the circumstances of the accident were "unclear".
The two men, who had been on a Rigid Inflatable Boat (RIB), were spotted in the water by the crew of the Isle of Wight car and passenger ferry, Cenred, shortly after 1800 BST.
The Solent Coastguard helicopter was scrambled but before it and other rescue teams arrived at the scene, the ferry crew had picked the men up.
They were then transferred by a waiting ambulance to St Mary's Hospital.