Depends where you are on the river. At the mouth it's salt. By the time your down town it's brackish. Further south it become's fresher.
Ton's of places to go by boat. If you trailer you can run the intercoastal one day and the river the next. Jax is mostly fish boats and cruisers but there are some performance boaters around.
Houses on the river are over 1MM and go up, way up. Even canal access is pretty pricey in most area's. I'm not familiar with condo pricing.
Come on down and check it out