What's your mast height?
I sail the St. John's often, but can't go south of Green Cove Springs because of a fixed bridge with max clearance of 35'. If you can make it to Jax under the bridge there's free parking at the Landing downtown (lots of bars and resturants) but no power hook ups, same at the municiple marina at Metro Park near the sports complex (without the bars and resturants). Other than that there are several marinas in the Ortega river just before you get downtown. I've never parked at one so can't tell you the cost. If you are retired, active or reserve military, the marina a NAS Jacksonville just past the I-295 bridge is very cheap with access to an air conditioned club house with grills, a kitchen, direct t.v., computer access, as well as, friendly locals with access to adult beverages.