BBC South Today producer Martin Webster said the plane was thought to have crashed in an open area at Throop village, near the Castlepoint shopping centre.
Shaun Spencer-Perkins, who witnessed the crash from Throop Mill, said: "I heard a rushing sound and I saw a plane about 15m above the ground racing across the fields.
"It impacted and bounced across the field, made it across the river.
"Members of the public jumped into the water to search for the cockpit.
"We waved down the helicopter, I took off my son's orange jacket and my wife's red jumper to get the attention of the helicopter."
BBC News - Red Arrow crashes during Bournemouth Air Festival
Unbelievably the Daily Fail is even naming the pilot...
before official confirmation.