The singer - who has also been a Coronation Street regular - said he struggled with "dizziness" as he stood on the 10-metre platform for the ITV show.
Duffy, who will be seen taking part in the live heat tomorrow, said he had already had painful mishaps as he trained for the programme.
"I have done a couple of belly flops, and they're really not very nice. It's like someone hitting you very hard with a wet fish. I don't want to do that again," he said.
Speaking about his struggles with the height, he said: "It's pretty scary, I can tell you. It reminds me of the early days with Boyzone when I did a bungee jump from 400 feet in Florida.
"The first feeling you get on the 10 metre board is one of unbalance and dizziness, to the point where you feel you're being dragged over the edge. You have to keep one hand on the rail to give you balance.
"That's an important part of the transition from being totally terrified. You have to get used to being up there. Although I heard the amazing Tom Daley say that you don't ever get used to it and that you always get butterflies in your stomach at that height."
Olympic bronze medallist Daley has been training the celebrities, with Duffy joined in tomorrow's second heat by Toyah Willcox and TV presenter Paul Ross among others.
Duffy said Daley had aided him by adopting the correct frame of mind.
"He's taught me to have a very positive attitude. I've learned that there's no point in taking on a challenge if you don't believe you will be capable of it. So I have to approach diving off the 10-metre board in a very positive frame of mind."