Submariners must interpret SONAR data and make rapid tactical decisions under challenging conditions. The broadband time-bearing plot is a common visualisation method which requires mental spatial transformations to generate an actionable representation. Participants (N = 81) were trained on a VR scenario that required them to spatially localize an enemy vessel indicated on a time-bearing plot. In the supported condition, participants were given access to a novel egocentric, conformal display, which wrapped the time-bearing plot around the user's field of view. The unsupported group received no assistance. Compared to a baseline unsupported condition, we found that the VR overlay markedly improved both speed and accuracy. Test performance revealed over-reliance in the supported group, but this phenomenon varied notably between participants. Those who jointly used the initial time-bearing plot and the conformal aid during training showed good performance with the aid, and no cost of prior exposure at test. Comprehension of SONAR data can be aided by novel display formats, but care must be taken to avoid over-reliance.