The way you describe is a good way of getting depths into RM, but it won't appear directly as a contour. Instead, the data will just be used as depth points in the same way as any other sonar log.
When you imported the contour with 0.5m depths, did you import it as a track in ReefMaster and, if you look at this track in the track edit window, are the depths correct?
How did you add the track to the project? You should add it using the "add to project" menu option, so that the depth points appear within the project tracks view. You should then generate the project, ideally with other depth points that you have gathered, and RM will create the contours.
Can you post a screen shot of the tracks project view with the imported track added?