Rescuers had urged Kevin Brady to avoid the section of the River Severn below Arlingham in Gloucestershire, saying it is "very very dangerous".
Whirlpools, moving sandbanks, sinking sand and no public access contribute to the risks.
"It puts a knot deep inside my stomach," the 32-year-old said earlier.
Geoff Dawe, from the Severn Area Rescue Association (SARA), said: "The noose is a horrible place, even in a boat."
"The big issue from my point-of-view is if he suffers from cold, and gets muscle cramps, then there's not much he can do."
The section is 4.2 miles (6.7km) long and Mr Brady is expected to take between one and two hours to swim it, according to Mr Dawe.
Two small lifeboats, deployed nearby for a training exercise, will remain on the water while he is swimming.
The fundraiser began his 220 mile (350 km) marathon swim in Plynlimon, Powys on 23 September and is aiming to finish on Severn Beach later this week.
He said he is attempting to become the first man to swim the entire length of the River Severn.
"When [Geoff] sat me down and said 'You shouldn't be doing this, this is far too dangerous, I completely advise you against this' I nearly wanted to cry," added Mr Brady.
"I'll be taking a look at the weather, chatting to people around us and we'll be assessing the risks as they come. That's the only way I can deal with it.
"If I start thinking too big and too much I just freak out a little bit."
A man bidding to swim the length of Britain's longest river has taken to the water on the notoriously perilous stretch called "the noose".