The soap star's murderous alter ego Tony Gordon ends up in the Weatherfield canal with Roy Cropper (David Neilson) next week following an after-dark confrontation between the pair.
O'Brien told The Paul O'Grady Show: "I'm not a great swimmer. You were standing in silt and you couldn't move. They were saying, 'Just pretend to swim'. I couldn't actually move my legs. I was glued!
"But we did the underwater stuff up at Fleetwood in a four-metre tank, and four metres is double the depth of an ordinary pool. Your ears go when you touch the bottom and stuff. I found it very difficult."
He added: "I have to say that David Neilson was absolutely brilliant. [But] I found it really difficult to hold my breath underwater."
Strait-laced café owner Roy has been aware of Tony's involvement in the murder of Liam Connor (Rob James-Collier) since last month.
Neilson recently told DS that the stunt filming "wasn't scary because it's all very controlled".