I suppose I am ancient! As I can recall the various places that Imperial Airways,Supermarine and RAF flew from.
The old man flew with IA and it was from Berth 101 and off the Royal Pier. The facility at Hythe was the maintenance facility.Supermarine from Woolston on the River Itchen.Where the Spitfire was designed and initially manufactured, RAF Calshot,my old "Alma Mater".Opposite between Hamble and Netley Follands had a slipway and serviced flying boats. Last but not least Saunders Roe at Cowes.Strictly not Southampton Water,they used RAF Calshot Tower.for takeoff and alighting.Also the Navy had a slip at Lee on Solent HMS Daedelous[spelling!] Hope that helps
I believe there was some sort of control from Dockhead,which is at the confluence of the Rivers Test and Itchen