'He pointed out vague shapes among the fuzz of the sonar screen.'

https://books.google.es/books?id=dhW9JFvD6LAC&pg=PT40&lpg=PT...

I think it just means the shapes of the objects outside the submarine that appear on the sonar screen.


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Note added at 52 mins (2019-08-30 20:14:58 GMT)
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Sonar (originally an acronym for sound navigation ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, communicate with or detect objects on or under the surface of the water, such as other vessels.[2] Two types of technology share the name "sonar": passive sonar is essentially listening for the sound made by vessels; active sonar is emitting pulses of sounds and listening for echoes. Sonar may be used as a means of acoustic location and of measurement of the echo characteristics of "targets" in the water. Acoustic location in air was used before the introduction of radar. Sonar may also be used for robot navigation,[3] and SODAR (an upward-looking in-air sonar) is used for atmospheric investigations.

Submarines rely on sonar to a greater extent than surface ships as they cannot use radar at depth. The sonar arrays may be hull mounted or towed. Information fitted on typical fits is given in Oyashio class submarine and Swiftsure class submarine.

https://en.wikipedia.org/wiki/Sonar

When the sonar screen revealed the conspicuous outline of a ship across the edge of the project area, new plans were made. The new work included additional high resolution sonar imaging, underwater video documentation over the wreck, a multibeam bathymetric survey, archival and photographic research, and limited diving around the area of the wreck (Hoskins et al. 2007b, 2007c).

https://www.thc.texas.gov/public/upload/publications/current...

You could also say 'outlines outside'.