Wake of the Red Witch 1948

This being a Republic Production means that the miniature effects were handled by the Lydecker brothers Howard and Theodore. The special effects credits are sub-headed under-water sequences although there are some miniature surface shots of model ships as well. 

The bulk of the work is indeed in an underwater setting with a miniature shipwreck teetering on the edge of a precipice which eventually slides over the edge. There is also a marionetted diver figure with subtly moving arms and legs along with the emission of a stream of bubbles.