Is it freshwater or saltwater in the area outlined in red (as I don't want to put my gear/ boat in saltwater)?

 

I stayed last year there. That area is fed by springs. Although you will run into some brackish water up river. You will find everything from snook, mangrove snapper, bass, mullet, manatees. Great area for fishing, terrible area for bugs, skeeters and noseeums will eat you alive.

There might be some very slight salinity, depending on tide...but not enough to worry about, hose it down, you will be fine. Have fun!