We tested a pretty quick but efficient method for getting a good idea about the layout of a wreck site last summer. It only works for fairly flat sites, though. We only tried this once on a short weekend project, but the results were quite promising:
1. Lay lines or tape measures across the site, in the same direction, so that the distance between lines is about 1-3m, depending on visibility. We only used two lines, but the wreck area was smallish. A bigger site might need more lines.
2. Mark the lines with arrows every 2m, numbered or otherwise easily identified markers would be nice but not necessary.
3. Photograph swimming above (1-2m) a line with wide angle (100degree non-fisheye is pretty good for this), shooting at a downward angle (about 45deg). You can't really take too many photos here, a photo/arrow is minimum, but a photo between each arrow won't hurt. Ideally you'll see the left and right lines at the edges of the photos.
4. Organize the photos so that they can be viewed quickly after each other.
The result is something like
this and
this. It gives you a nice "diver's perspective" view of the site, and is quite quick to perform. It has some advantages over video:
-more detail
-a lot smaller file size
-easier to move back and forth
-very few video housings have wide enough ports/lenses
Obviously the lines will also help with drawings and mosaic photography, but those methods usually require somewhat more bottom time and a lot more postprocessing to get presentable results.