Last August Mr Boanas entered the history books when he emerged exhausted and mud-splattered from the river and became the first person in living memory to cross on foot.
Now he will don a dry suit and helmet to plod across the bottom between the tw
o towers of the Humber Bridge in the pitch-black murk at depths of up to 32ft.
His summer effort raised an amazing £43,000 for the charity DebRA UK, which helps people with epidermolysis bullosa (eb), a painful genetic skin blistering condition – and he wants to raise even more.
And Graham, who is already training hard, admits he enjoys the challenge, and said: "I'm passionate about the kids who are suffering from eb. I always like to do something that is totally unique and I think that's what people want."
This year's challenge will take place just a week after he crosses a one-and-a-quarter mile stretch of the river Mersey on August 13.
Graham, who owns Hull-based electrical firm Power Plus, will be trained by experts from the city's Northern Divers, who will be teaching him how to plough along in the dark without being swept away by the strong currents.
Northern Divers managing director Nigel Thacker said: "The problem he will have is nil visibility with the tidal flow. It'll be done on a neap tide, but where the crossing is going to take place there is little or no slack water.
"One minute the flow is going one way and the next it will change and go the other direction."
Keeping him down will be a vest with lead weights and he will be fed with air via an umbilical cord.
Mr Thacker said: "I think it's a bit harder than he thinks – as soon as he goes below he might as well close his eyes."
He is also hoping Top Gear presenter Jeremy Clarkson takes up a challenge to race him. The idea is that while he plods along underwater, Mr Clarkson will drive from one tower to another – without using the bridge but via Goole.
The pair met when Mr Boanas was in the audience on the set of his BBC2 show.
Graham survived his titanic struggle with the river – his size helping him just keep abreast of the water – on his crossing in August. He returned to a hero's welcome, with some 700 people lining the foreshore. His team are looking for sponsorship and can be contacted on Hull 221552.
