For your info. The 4 Yellow marks of Eastney beach, mark the Launching/Windsurfing Area By ST Georges Road. Steer clear of that if you are towing.
However, Any area outside of langstone harbour is good for towing if the sea conditions are ok. I would keep clear of the Langstone Run very strong tidal currrents & lots of Jet Ski's.
Watch out for the East Winner Sand Bank as well
When out with the toys I run along the Seafront of Eastney towards Southsea keeping 200m offshore and watching out for anything else coming my way
Hope this is useful.