There is lots of sailing to be found on the St John's. It is pretty wide open just past the city and down to Green Cove Springs (about 30NM's if my memory is correct) but you have to deal with currents.
Imagine 2 Frolic can add a lot to this discussion as he is currently docked in Green Cove.
Very limited or no sailing available on the ICW between Mayport and St Augustine.
The Mayport Inlet is easily navigated but marias and dockage are very limited in and around the inlet.
St Augustime inlet is a bit more of a challange but lots of services in that area.
Other with more experience can add to this and will correct me if I am wrong.
Courtney is My Hero
If a man is to be obsessed by something, I suppose a boat is as good as anything, perhaps a bit better than most - E.B. White