Like Vasco said. No Problem. The river traffic is not bad at all. I have never had any problem with the
GPS at Mayport. I travel this area allot. Plan on at least 10 hours to get to Green Cove Springs. I would suggest staying at the City Marina or anchoring behind Exchange Island. Out of the way and good holding.
The river narrows in downtown Jax. The current rips through there at about 3 knots. Try to go with the tide. There is one lift bridge in Jax. The Main Street Bridge which opens on the hour between 7:00 PM and 7:00 PM weekdays . After hours it opens on demand. The rest of the bridges are all at least 65 ft. Past Green Cove the bridge is 45 ft with an additional 2 feet in the middle.
Once through Jax the river opens up to 3 miles wide average depth in the channel is 13 ft. If you get out of the channel it's usually no problem as the bottom is soft mud.
If you need more help call me at Green Cove Springs Marina and ask for Capn Dave