Morituri 1965

Almost all of the ship action in the movie was shot on a real freighter but there is a cutaway to a tanker on fire and exploding at night which is a miniature. Another exploding ship miniature can be seen as a rear screen projection from the bridge in daylight and as a cutaway.  A still of a tanker model attributed to Morituri can be found in an old studio auction catalog as lot 855. 

There are a couple of stock miniature shots (lifted from older movies) of a submarine and a close up of a torpedo being fired. There is also one stock shot of a convoy in miniature covered in a layer of added fog.

Stock submarine shot.

Stock convoy miniature shot with added fog.