As the song was written during Cyril's time in the submarine service and was about a submarine (Dabtoes forrard and the dustmen aft), then in, the context of the song, chicken on a raft was fried egg on fried bread.

The galley on a diesel submarine of any class is tiny. There are no facilities for toasting bread in enough quantity to provide for the entire crew. The chefs, therefore, had to deep fry the bread before topping it with a fried egg to produce chicken on a raft. It may have been different on an aircraft carrier, I don't know, I had no experience of those.

During the same period he wrote in Diesel and Shale: 'The big man in Dolphin, he sent for me'. Any Royal Navy submariner could tell you that that refers to Flag Officer Submarines (FOSM), who up until fairly recent times dwelt in HMS Dolphin. If you was so inclined, you could argue that Cyril was referring to a man of above average size, but you'd be wrong.