First off, cool that you all found my boat ramp map again. I made it for one of my company projects. It is up to 11,000 views. Here it is for those that have not seen it:
Boat Ramps and Marinas on Old Hickory Lake, TN
And my steam plant picture map from past years. Glad it is still being used.
As for getting to the steam plant from Coles Ferry. It is no problem getting there. Just ride the current to the mouth while staying to the inside of the channel. The issue is getting through the shallows between the islands coming home. I know the kayakers make it just fine, but depending on how heavily you load the jonboat and if the water is falling, it may be hard to get through.
It is no problem to move around behind the islands and the fishing can be good up there. The deeper water is near the shore away from the islands and it is 2 to 4 ft deep. It is usually around 1 foot deep between the islands at the deep spot, but that can vary 6 inches either way with the falling or rising water. Six inches is fine for a kayak, but not so sure about a loaded jonboat.
It has been a few years since I used the pathway between the next islands upstream (on right side of picture), but that used to be deeper and bassboats could run it on a plane. So you almost always had at least 1 1/2 ft of water. Plenty for your jonboat. It is not that far to trolling motor up to the upper pass. I do it all the time looking for shad on the flats. So you could probably get around that way if you needed to.
If you try this, bring a long rope so you have it if you need a tow and don't leave late to return to the ramp your first time, so if you need help there will still be boats around.
Stay safe,
Jim