Floating the Hi
You have been told right. The Hiwassee is a great river to float, and it can be treacherous.
LP boats are more likely to take on water on the Hi. With their low cut they tend to take on more splash from white water. It can be done in a low profile, just may need to bail a little water after dropping a few places.
There are three put-ins and take outs on the upper section of the river from Reliance to the Powerhouse all of them being public. There is one in Reliance, one at Towee Creek and one at the Powerhouse.
The float from the Powerhouse to Towee is a relative short float (+/- 2 miles) but is the easiest to make. Fox's is really the only white water you will face and as long as you follow the heaviest flow of water you will do just fine.
From Towee to Reliance is about a +/- 4 mile float. The river from here takes on a different character and becomes more turbulent and rocky. Towee is a good boat banger and soaker if you don't line up right as is Big Bend, the Stair Steps, and Devil Shoals.
Big Bend is a place that will hang your boat up as is the Stair Steps. It is a bit easier to get your boat unstuck in Big Bend than it is in the Steps. The Steps is the place that gives most people fits and it has also claimed a few boats over the years. The line through it can be difficult and if you get stuck, becoming unstuck can too be a bear. Devil Shoals is not so hard to navigate but is a place that is capable of swamping a boat because of the amount of water that rushes through it. I remember when rowing the LP that this would be the drop that would take on the most water. Heck I still take on water sometimes making this run, just not as much as I use to.
The best advice I can give is this. I would go out on low water and survey these areas and make sure you are comfortable with them and see what lies beneath the water before TVA turns on the wheels. Then I would run the upper section and take out at Towee before doing the whole section of the river above Reliance. This would allow you to gauge how comfortable you are with the river instead of trying the run the entire run at once.
Oh yeah, make sure you have all required PFD's on you as you float. They will ticket for this if you do not have them.
You can't depend on your judgement when your imagination is out of focus. - Mark Twain