Mr Boanas, 43, from Swanland in Hull, who is 6ft 9in tall, became the first man to walk across the River Mersey – with his head sticking out above the water.
He also performed a similar feat on the Humber and then capped it by walking under the same river, this time on the bottom, using a breathing tube.
All the money raised has gone to DebRA, a charity that helps children with skin-blistering condition epidermolysis bullosa.
Now he has announced his next and final challenge – to walk across three rivers in one day in August next year. He wants to cross the Mersey, the Thames and fly back home to cross the Humber.
However, the details have yet to be confirmed as Mr Boanas has to get permission from the Port of London Authority, which controls the Thames. If he cannot get permission to walk across the Thames, he will attempt the Severn. None of the crossings will be underwater.
Mr Boanas, who has not set a fundraising target for his final challenge, said he was delighted to have reached £100,000.
He said: "It's all thanks to the sponsors and people who have done so much for me.
"Three Rivers will be my retirement challenge. There isn't much left to do now. There are technical obstacles and it's still in the early stages, but it will be done."
Tony Eckersall, DebRA fundraising manager for the north, said: "It is a fantastic amount of money for a small charity and we are extremely grateful for all the work that Graham and his wife, Jan, have done."