For the most part the HMS Amethyst of the story is represented by the full size vessel herself and by a sister ship HMS Magpie for all the sailing and shots of the guns firing. There is also a large amount of full size and impressive pyrotechnics going on around and onboard the fullsize ships.
For a couple of scenes however the Amethyst is represented by a model supervised by George Blackwell.
One scene is of the ship passing a floating boom gate surrounded by partly sunk ships, the other depicting the approach of the searchlight encrusted gun batteries at the mouth of the river.
There is also a brief shot of a merchant vessel with festoon lights on fire in silhouette which for most of the scene is a full size vessel but one passing shot appears to me to be a model.