I "sailed" the St. Johns in Florida some years ago downstream (northward) from Sanford to Jacksonville. I found I had to motor most of it, for often regardless wind direction the trees along the river influenced it so that most of the time I was heading into it. That, and the fact my direction was constantly changing with the river made putting up sails almost useless. It was better when crossing many of the lakes along the way, and when almost to Jacksonville it opened up enough to do some sailing. It was a wonderful trip though for beforehand I had studied the history of it, and unlike offshore sailing there was always a shoreline with the flora and fauna, along with oxbows, islands, and gunkholes to explore.
Have fun!