From Wikipedia: The St. Johns River is the longest river in the U.S. state of Florida and its most significant for commercial and recreational use. At 310 miles (500 km) long, it winds through or borders twelve counties, three of which are the state’s largest. The drop in elevation from the headwaters to the mouth is less than 30 feet (9.1 m); like most Florida waterways, the St. Johns has a very slow flow rate at a third of a mile an hour (0.2 km/h), and is often described as “lazy”. It is one of a small number of rivers in the United States to run north. Numerous lakes are formed by the river or flow into it, but as a river its widest point is 3 miles (4.8 km) across, spanning several miles between Palatka and Jacksonville, the latter being the largest urban area on the river. The narrowest point is in the headwaters, an unnavigable marsh in Indian River County, Florida. In all, 3.5 million people live within the various watersheds that feed into the St. Johns River.
As we entered the St. Johns River, the first site is a commercial shipyard on the northern bank laying alongside the entrance to the continuation of the ICW heading north. The shipyard is always busy with megayachts and commercial vessels in various stages of construction and repair.
Turning upstream toward Jacksonville (westward, at this point), we cruise past industrial areas with container ships being loaded and unloaded, and other port facilities catering to ocean going ships of all types and sizes. There are also beautiful homes on bluffs overlooking the broad river as it twists and turns it’s way northward from it’s swampy beginnings 300 miles upstream.
About 20 miles upstream of the ICW, downtown Jacksonville comes into view. We take a spot at the free dock at Jacksonville Landing, and enjoy the busy downtown area for a day. 
Jacksonville Landing is a great spot to spend a night or two and take in the attractions of downtown Jacksonville. Those include a library, art museum, free trolley, monorail to the other side and the Science Museum and more restaurants.
We depart Jacksonville Landing the next day, Wednesday, May 19th, pass under the I-95 highway bridge and the CSX railway bridge and onto one of the large “lakes” that form various parts of the St. Johns River. This is where the St. Johns River curves 90 degrees and becomes essentially a north-south river the rest of the way to it’s origins. At this point the river is 3-5 miles wide at various places, so there’s not much to see on the shore line. There are also very few cruising boats, but quite a few smaller fishing boats heading upstream to their favorite fishing grounds.
We stop at Mulberry Cove Marina at Naval Air Station Jacksonville for a few nights, and then cruise a short distance further to Doctors Lake, a nice anchorage on the western side of the river where we spend a few nights enjoying the sunshine and mild (not hot – yet) temperatures. Judy even goes for a swim in the lake. We’re the only boat anchored in the Mill Cove on the southern shore of Doctors Lake, but there is a boat launching ramp about 1/2 mile distance with many PWCs and small fishing craft being launched throughout the day.
We returned to Mulberry Cove Marina for a few more days while Judy drove home for a few days, then departed southbound on Thursday, May 27th heading upstream again. That day we departed the St. Johns River for a side-trip up Black Creek just north of Green Cove Springs. The creek is a narrow throwback to the Florida of yesterday, meandering through thick cypress swamp on both sides. We turned around a short distance before a 20 foot high bridge that would prevent us from going further, worked our way back 
downstream seeing an occasional bass boat flying by on its way to a favorite fishing spot at 60 plus miles an hour, then re-entered the St. Johns River and anchored for the rest of the day at Hallowes Cove (approximately statute mile 45 from the mouth of the St. John River). We took this opportunity of an early afternoon anchorage to put the dinghy in the water and make a short dinghy trip around the cove, and to take a few pictures of Sanderling at anchor.
The next day (Friday, May 28th) we move further upstream to the small town of Palatka at approximately statute mile 77 where we tied up to the town dock in one of the two locations on that dock which can accommodate a boat the size of Sanderling. Most of the dock, on both sides, is taken up with short slips that can be used by boats of up to perhaps 24 feet in length. Unknown to us when we arrived, the Memorial Day Crab Festival was taking place, and the dock was in the park at the south end of the area where the festival was located. Helicopter rides were a hot item, and the small helicopter landed in a parking lot at the land-side of the dock about 100 yards from Sanderling every 2 minutes and 15 seconds – cost $10.00 per person! The flights started at 10:00 AM and continued until 10:30 PM – needless to say, the noise never stopped, but it was fun watching the bird load and unload its passengers (two at a time) and observing the small boats coming and going to the small slips throughout the day. A boat launching ramp alongside the dock added additional viewing enjoyment! We walked the streets of the festival one hot afternoon and sampled some of the food being offered. After two days, it was time to depart even though the festival still had a day and half to go!